However, not every catastrophic event in a character's life has to be ultimatized, especially ones that didn't pan out well in the end. Many times in these mainstream comics, some big events that become unpopular are somehow phased out or retconned. If they decided to make Nightcrawler's dad Satan in the UU, and then realized that the idea received negative reactions for both universes, they'd be screwed. So it might be better to use what worked in the past, and etch out their ideas from that.
The problem will arise when the new events become the status quo of the character. If Wolverine ends up leaving the X-Men forever and is an Avenger for the rest of his comic book career, then UX would have to incorporate this. That shouldn't be too much of a problem (obviously BKV can write the X-Men without Wolverine). Same with 'Sins Past'. If Stacy's kids become popular characters, a writer will find some way to incorporate them into the UU (Stacy's cousins, Stacy's half-brother/sister who grew up at an accelerated pace because of super-serum, I really couldn't care less about these two).
There are many stories that can be told in the Ultimate Universe, many characters who have tales not yet covered. I don't see USM lasting for 500 issues, but if there is a point where Bendis (or whatever writer for whatever title) has ultimatized every great story to their breaking point, well then, there are twice as many crappy stories out there that could easily be fixed up. The ideas won't run out.
