Big centralized exchanges that only offer crypto to crypto will eventually die. There just isn't a need to trust anyone in crypto and people will come to realize that very soon with all the DEXs coming out. One of them will do it right, advertise enough to get people in, generate hype and good to go.
We'll need the fiat to crypto exchanges because it's a way for new people to get in and is highly regulated, and we'll need DEXs. Polo, binance, bittrex, they'll either need to become decentralized, which I think binance said 3y where going to, or they'll just get rekt.
Yeah, we cannot really do without fiat exchanges anyway considering what you have said and also considering that for now, there is absolutely no way to even spend your crypto without having to convert to fiat first and I am sure the government is trying to make sure that this comes to play first before the whole crypto space explode and they are not even able to easily handle it based on their regulation.
I am very much confident on bittrex to survive and obviously since they are apparently complying with all the SEC rules and regulations, I am sure they will be one top notch US exchange to use. I am sure some exchanges may end up being blown out and most especially the shitty ones, but I guess time will tell, but at the end, it also depends on where the exchanges are situated anyway, but there are just some of the big exchanges that I am sure will keep up.