I do not think that Vault-Key slottable alternate combo kills are in any way healthy for a format. They exist in Vintage because the format is super-broken. I do not think that kind of "Oops I win" play pattern coming out of decks that are otherwise not combo decks is in any way, shape, or form healthy for a non-Vintage format to have. This isn't just cute interactions that get a lot of value, this is decks w/ 2 card infinite combos that don't even need to play those combos to win.
I don't have an issue with Infect as a deck- I just suspect the 1 drop is going to get axed eventually because as they print more and more pump spells they'll need to notch it down a tad.
In Legacy, you can tap out into combo decks and deploy threats against them without an issue. Where Twin deviates from other combos is that it deploys its pieces during your end step while denying your opponent resources on your next turn.
End step interactions just suck and aren't fun to play against. I think they're the most miserable thing in the game, but they're a necessary evil we're stuck with due to the way the rules played out. Decks that are able to operate on the end step essentially get to Time Warp on your end step. It's a big reason that people hate playing against control decks, because they end up with a "play a spell? Counter! Don't play a spell? Draw cards!" play pattern that isn't fun at all. (See: STD Faeries, quite possibly the most miserable deck in existence.) This is why, back in Odyssey block, they tried to make all card draw sorcery-speed to force those decks to actually have to make choices during their turn other than "wrath, Exalted Angel, or pass?" They've gotten away from that recently, and I think it's a mistake. (See: Revelation, although I don't even think Revelation was the biggest issue.)
Legacy combo decks have to be all-in on their combo, and play out their combo directly on their turn. They don't get that "free turn" thing beyond getting to set up with stuff like Brainstorm, or possibly tutoring. This makes them much less frustrating, as you know when "danger time is" opposed to every single one of your turns being "danger time".
The "cards keep getting added" is why you ban the actual combo engine instead of the cards that tip it over the edge. It's why you ban Pod instead of Siege Rhino, Survival instead of Vengevine, and in this instance, Twin over Exarch. The game will keep expanding, and once something crosses that threshold, you have to nuke it, because it will just go past it again later if you try and wuss out on it. They tried that approach in the distant past, and it failed miserably. The only format that gets that approach nowadays is Vintage because it doesn't play by the rules of other formats.
I'd be totally cool with putting Jace in MM2 or 3 and timing an unban with that, just to see if it can work or not.
