Its hard to guess about something that doesn't exist at all right now, but exchanges wouldn't be moving offshore to save themselves a ftt, they would do it to save their customers a ftt, and in turn, their business.

Again its only speculation but I'd imagine traders who are effected would likely end up being the sole US employee of a shell company in whatever tax preferential country the exchanges land in. So the government would get theirs, but it would just be taxes on income.