South Korea just co-hosted with Japan. FIFA would want to avoid that, I think - same reason I don't consider especially Germany and to a lesser extent France as possibilities in 2022, plus of course Russia (a European nation) hosting. If you're changing the rules and traditions, which would become more likely if the WC is cancelled closer to its opening, you could theoretically do it anywhere with the stadiums and the infrastructure to handle millions of visitors in a short period of time - the US, England, Germany, France, probably Spain and Italy (?).
I don't think the fact that the US government is doing the prosecution would count much, generally speaking. Terrifyingly, America's court system has a relatively good reputation for professionalism and fairness - it's not that the US is so great, but everyone else is even worse. Plus, I think the fact of the broad influence of American financial institutions makes the US a natural candidate to pursue the case.