I think that shitcoins will never completely disappear. The cost of producing them is negligible, but the utility is quite high (for the one who produces them). The motivation will always be there, and as long as an entity has a localized monopoly or strong network effects, they can easily create a new shitcoin that will be economically relevant for some period of time. This isn't an argument in favor of shitcoins. The existence of bitcoin will serve as a constraint on shitcoinery, but I doubt the latter will go away entirely.

No. There is a reason to think that USDT has more adoption. But it's users can't save money so their only option becomes Bitcoin.