I can't speak to the XFL, I don't follow it, and won't unless they add a Philadelphia team, but while the USFL could potentially fold, it doesn't make sense to me. It's owned by Fox, lock, stock, and barrel, and is a relatively cheap way to generate a great deal of content for their network (split with NBC at the moment, but this is the last year of the current media rights deal, and it's possible that doesn't get extended.)
To me, the biggest flaw for the USFL is that they aren't in all 8 cities yet. I would love to get season tickets, but I'll be damned if I fly to Detroit to watch the Stars, so they need to get their butts back to Philly (Or, if they have to do hub cities, pairing Pittsburgh and Michigan, and finding somewhere in the rough vicinity of Trenton or Princeton for the Stars and Generals, would at least entice me to go see one or two. That should be roughly equidistant between where the two teams would hope to end up.) Once they have the opportunity to build up a proper local fan base, we can see how the league is really doing from that perspective.