So here's the thing...
Stoneforge Mystic should come off the Modern banned list. And while that sentence might sound terrifying, I think most people thought Jace, the Mind Sculptor was an even scarier card to cut loose. And look where Jace, the Mind Sculptor is now! Not only does Jace, the Mind Sculptor see less play in U/W Control or Jeskai Control than Teferi, Hero of Dominaria, but it just seems the format is too fast for it to even be viable.
It's possible that Jace, the Mind Sculptor seeing very little play is a product of tools at your disposal. After all, not having access to something like Force of Will or Mental Misstep means protecting yourself while you tap out for Jace is pretty difficult. Sure, if you get to untap with a Jace, the Mind Sculptor, you should be in good shape, but that's true for just about every Planeswalker ever printed. Getting two or more activations out of a Planeswalker is what every deck playing one of those cards dreams of. It's where you start to gain an actual advantage from your spell as opposed to just having it die and (usually) replacing itself.
But we're not here to talk about Jace, the Mind Sculptor. We're here to talk about the health of Modern, and the implications of what unbanning a card like Stoneforge Mystic could potentially do to the format. So, let's begin in earnest and discuss exactly why I think Stoneforge Mystic is more than fine for Modern.