As great as I think our deck was for the World Championship, and as much as I think it's a fundamentally sound, powerful deck, it was ultimately a metagame deck, designed to prey on midrange green decks that cast two- to five-mana spells at sorcery speed, an aggressive red deck that punishes blocking and "destroy" effects, and control decks that were unprepared to fight over Search for Azcanta. That's the precise field we expected and the precise field we encountered, but it can't be the future.
The decks are exploitable.
Despite Huey's win, Temur has an awful U/B matchup. Ramunap Red is designed to punish people who are trying to block, with Hazoret the Fervent as a payoff because it's so hard for many opponents to answer. Against U/B, it's just an underpowered creature deck. The burn spells aren't good because the U/B deck starts gaining life with its removal spells going long, Hazoret isn't good because being indestructible doesn't prevent it from being countered or exiled, and while haste can be nice, it's not actually that useful against a deck full of instants. The result is that it's just a red creature deck, and red doesn't even have the best aggressive creatures.
The fact that U/B beats the other decks at the World Championship doesn't mean we have a three-deck format; it means we have quite the opposite. If that were the case, this would reduce to a one-deck format, but Standard offers a fairly wide range of options for beating U/B Control. You just have to consider its vulnerabilities.
Take a look:
First off, U/B Control can only answer resolved creatures, planeswalkers, and lands. If you can resolve an artifact or enchantment, it'll be on the battlefield for the rest of the game. There are cards that U/B can play to get around this, like Commit // Memory or Consign // Oblivion, but those cards don't fit its fundamental gameplan well, so they shouldn't be expected in large numbers, if at all. U/B is looking to interact with artifacts and enchantments with counterspells, or discard after sideboarding, but this means that they don't always have a chance to interact with cheap artifacts or enchantments, and sometimes you can sneak through more expensive ones.
Some examples of cards that are huge problems for U/B Control:
This is my top tier because they come down before U/B can counter them if you're on the play (Spell Pierce exists in the format, but it's a horrible strategic fit for the gameplan U/B's trying to play, so I don't think it's even a future concern) and they don't require anything else to go right. If you cast one of these, it will substantially increase your chances of beating a U/B Control opponent.
Not quite as good because they're more expensive or require other things to resolve:
This isn't an exhaustive list. There are some more expensive cards that it would be extremely hard for them to beat if they resolved, like Metallurgic Summonings, but these are some of the most likely problem cards one could try to find a home for to exploit this weakness of U/B.
It's important to understand that a lot of these won't win the game outright. U/B has trouble answering them, but it doesn't always have to. The Scarab God is an absurdly powerful finisher, and if the opponent sticks something that will win the game eventually, they're just forcing the U/B opponent to try to kill them. If your deck doesn't have any creatures, The Scarab God is often "just" a 5/5 creature that's difficult to kill, but that's still a fast clock, and if you do have creatures, things get worse quickly. If you're planning to beat U/B Control by gaining a small edge over time with an artifact or enchantment, you need to make sure your deck is well prepared to answer The Scarab God.
Ixalan's Binding really shines here, since the U/B deck has such a small diversity of threats, if you can answer their first Scarab God and stop them from casting the second, you're remarkably close to making it impossible for them to kill you. (Of course, they can adapt to this by adding Consign // Oblivion and returning their exiled The Scarab God to the battlefield at an inopportune moment.)
Basically, if you have a deck with a good control shell and some countermagic or ways to exile creatures, you can likely beat U/B Control going long by threatening them with the right artifacts and enchantments, but that's not the only way.
U/B Control, as built for the World Championship, has no sweepers, and the sweepers available to it, Yahenni's Expertise and Bontu's Last Reckoning, aren't very good. This means the deck has to interact on a one-for-one basis. This makes tokens extremely effective against them. I've already mentioned Hidden Stockpile, but cards like Servo Exhibition, Sram's Expertise, and especially Start // Finish (because it's an instant) create threats they have a really hard time stopping. Again, this strategy simply forces them to rely on the power of The Scarab God.
Before the World Championship, I asked Josh Utter-Leyton what our plan was for beating Thopters made by Whirler Virtuoso. His answer was "Ignore them," and that worked really well. Tokens are hard for U/B to answer, but they also don't necessarily represent that fast of a clock, so the U/B deck can just go over them.
Of course, in the case of Thopters made by Whirler Virtuoso, The Scarab God has a lot of creatures in the graveyard to play with because Temur plays so many creatures that they have to cast them, and many will get countered or killed with Fatal Push, which allows The Scarab God to take over a game quickly. In the case of the Hidden Stockpile / Anointed Procession control decks, The Scarab God is remarkably ineffective at dealing with the tokens made by Hidden Stockpile because it won't have other creatures to turn into Zombies, and as it's lacking any sort of evasion, it's often relegated to simply blocking one token every turn.
So again, the effectiveness of hard-to-answer threats really comes down to your ability to outpace U/B's efforts to kill you as their solution.
Another way to exploit U/B Control looks to exploit the same weakness in a different way. Because U/B doesn't have a sweeper, it lacks a catch-up mechanic (again, outside of its creatures). Its plan is to use Fatal Push and two-mana counterspells to avoid falling too far behind in the first place. A creature will often slip through the cracks in the early-game and deal two damage a turn or so until U/B has caught up and found a removal spell for it. Then they'll be at around eight life, but they'll have complete control of the game.
I mentioned that Ramunap Red doesn't have the best aggressive creatures. If it doesn't, what color does, and why are the Red aggressive decks the most successful?
Against U/B Control, black has the best aggressive creatures. They hit harder and cost less mana than red's creatures. The red creatures are good because they prevent opposing creatures from blocking, which is necessary against all the green creatures in Standard, but if you're trying to attack against a deck with no creatures that cost less than five mana, you don't want to be spending mana on creatures that are good against blockers. Black's creatures cost less and hit harder. Dread Wanderer, Night Market Lookout, and Vicious Conquistador are all one-mana threats that attack for an actual or virtual two, and Dread Wanderer is particularly hard for U/B to deal with.
Back those up with two-mana threats like Scrapheap Scrounger and Glint-Sleeve Siphoner and the low curve of resilient creatures really punishes U/B's more expensive answers. If, instead of falling behind by a Servant of the Conduit, they fall behind by two two-power one-drops, they're taking four a turn instead of two and need twice as many removal spells to catch up.
The problem with the aggressive black deck is that these creatures are horrible against other creatures. They may "attack for two," but Vicious Conquistador and Night Market Lookout get brick-walled by a Servant of the Conduit because they don't actually have two power.
Another effective strategy against U/B control is the juke. In sideboarding, U/B Control is looking to make sure it has the right kind of answers to the threats it's being presented with. This means cutting Fatal Push for Duress against other control decks, for example. As U/B tunes in sideboarding to prepare for very specific fights, they can be beaten by presenting an angle they're not prepared for.
In the World Championship, I lost to Kelvin Chew despite feeling like my deck was better-prepared for the mirror because his sideboarded Gifted Aetherborns proved remarkably problematic. Given that he had very few targets, it didn't make sense to leave Fatal Push in my deck, but that meant I didn't have a good answer when he cast one, and while it wasn't a fast threat, it put me out of position and forced me to act first, something you never want to do in a control matchup.
If he'd had a creature designed for that matchup in particular, like Glint-Sleeve Siphoner or Dreamstealer, it'd have been even harder for me, and I think we'll likely see U/B Control decks dedicating more sideboard space to creatures to attack the mirror as the format progresses.
The question for the upcoming Standard tournaments, like US Nationals, isn't whether U/B can be beaten; it's how to do it without folding to the other decks you'll expect to face, like Temur, Ramunap Red, and likely Hidden Stockpile decks that are looking to beat U/B Control while having a decently proven history on Magic Online of having game against the field.
Personally, I'd love to believe U/B still has the chops, that people won't find the right tools to beat it without losing to badly to Temur. Of course, the other question is, with Temur winning the World Championship rather than U/B, will people even try that hard to beat U/B, or will everyone still focus on beating Temur? If people are sleeping on U/B, I think it's a great choice for this weekend, but I think people like to find and play new decks, or old decks that might be better-positioned, and I think people will show up with a variety of decks that are very hard matchups for U/B control.
It might seem weird to list U/B Control among the decks I'd be afraid to face if I played U/B Control myself, but I think it's possible to attack the mirror in a lot of different and very effective ways, and I'd generally be terrified that my opponents had a sideboard plan that was considerably better than mine, especially if I was worried about having the tools to beat other decks and feared that they might overload on preparation for the mirror.
God-Pharaoh's Gift hasn't gotten a lot of attention lately, but I think any proactive U/B deck can be a horrible matchup for U/B control with the right sideboard because they have access to cards like Duress and Negate to support Arguel's Blood Fast, which is much stronger out of the proactive deck than the control deck. God-Pharaoh's Gift in particular is scary because the card itself is so hard for U/B to beat and they can force through a Gate to the Afterlife activation with Duress, Kitesail Freebooter, and Negate. The Scarab God is incredible against them, but you won't always draw it and they can answer it.
In any case, I think this Standard format is far from solved, and I look forward to the innovation we'll see at US Nationals this weekend.