Though, you can mulligan aggressively for Leyline, and if you hit it you win. It's not great, but it might happen often enough to make other decks viable
Hmm. That sounds good enough to actually give the Poro deck a low win percentage. But then if the Poro deck isn't very popular, people won't play the Leyline mainboard, so Poro will get more free wins, which will increase the win percentage.
We could say for sure though that this OP card would be the kind that polarizes the format around itself like Skullclamp did; you either play with it or against it.
It'd also be Restricted in Vintage, making it useless. :D