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I've been fascinated for a long time with the idea that, at some point, it will not be right to play exactly 60 cards in Constructed, and when that time comes, no one will have any idea how to build a deck.
The question current deckbuilding looks to answer is, "Which are the best 60 cards?" Whenever something is added, something has to be removed. This simplifies the process of deckbuilding in a way that I think is really important to making it manageable. Once you remove that restriction, how do you figure out how many cards you should play instead? When you add something, are you supposed to remove something? The task becomes much harder.
The typical reply to this is that it doesn't matter because 60 will always be right, or at least it'll be right often enough that considering alternatives isn't worth the time. The most obvious and frequently used argument for playing the minimum number of cards is simple: some cards are better than others, some cards will be your best cards, and you don't want to reduce the frequency with which you draw those cards. This argument is genuinely sufficient by itself most of the time, but there are other arguments that are even harder to imagine exceptions to; this argument depends in some ways on the card file available, but the others are closer to mathematical facts.
The biggest is sideboarding. You can't increase the size of your sideboard with the size of your deck, so as you add more cards to your deck, you reduce the impact your sideboard cards can have; you get to change a smaller percentage of the cards in your deck and the most important cards you sideboard in will show up less. This is pretty similar to the first argument, that you'll draw your best cards less often, but it highlights that it's even more likely that you have some cards that are wildly better than your other cards in post-sideboard games.
Another big mathematical hurdle is that there's actually a difference in the consistency of your draws between a 60-card deck with 24 lands and a 600-card deck with 240 lands. With a 60-card deck, if you've only seen two lands in your top ten cards, there are 22 lands out of 50 cards remaining, so rather than having a 40% chance to draw a land, you have a 44% chance, but with the 600-card deck, you have a 40.3% chance.
Basically, your draws are forced to balance out over time as you get through your deck because the ratios remaining in your deck change throughout the game, but that effect is minimized with a larger deck. In this case, that's an extra land drop we'll miss almost 3.7% of the time. Fortunately, that difference is much smaller when we're talking about moving from 60 to 70 cards than when we're talking about moving from 60 to 600 cards.
So, given those arguments against, what are the arguments for that could ever get us there? The most obvious, but also worst argument: in Magic, you lose when you run out of cards. The more cards you have, the less likely you are to run out.
This almost never matters when neither player's gameplan is to attack the other's library, and when that's happening, the difference in size small numbers is only a difference of a turn or two. Yes, a full-on Battle of Wits deck will have a great matchup against a Sanity Grinding deck, but we'd need decks like that to be far more popular than they are for this to be a serious consideration.
Another common argument is the toolbox argument. If your deck is full of cards like Fauna Shaman and Chord of Calling so you have a lot of access to single copies of utility creatures in your deck, there's a lot of extra value to each extra bullet you can fit, and on top of that, the more narrow bullets you put in, the less you want the odds of accidentally drawing one of them to be, so the more cards you have, the less likely your are to draw a narrow bullet that you're hoping to only see when you search for it.
The problem with this is that you generally can't afford to put more than a few bullets in your deck to begin with. If you have ten bullets in your deck, even if you're playing 70 cards and viewing them all as "bonus slots" that "don't take away from your other cards," you're still one-seventh bullets, meaning an opening hand will have an average of one bullet, which is like taking a mulligan every game if you assume bullets are narrow and not worth a card in the wrong place. The result is that these decks tend to choose one to three good, versatile bullets and relegate the others to the sideboard, and those one to three cards don't really justify playing more than 60 cards.
The argument that I think is most relevant to Modern, the format where I think it's most likely that playing more than 60 will be right first, is a variation on this argument that focuses on fetchlands specifically.
A strong case can be made that fetchlands are the most powerful cards in Modern, especially if we set aside a few possible exceptions like Mox Opal or maybe Lightning Bolt. To begin with, they're lands that, with proper deck construction, function as any color of mana in our deck, like Evolving Wilds, except that they usually function as any two colors in our deck, and they don't have to enter the battlefield tapped.