This is one of the those topics that's uncomfortable for U2 fans to talk about (as evidenced by the lack of responses). At this point in time, with the guys hitting 50 in a couple of years, I really don't think "U2" as such would survive the loss of any member -- Bono has basically said as much in the past. Of course the guys would still be involved in music, but at their age, with their legacy now, there's no way they're going to just replace Edge or Larry -- it's unthinkable.
Clearly Bono, as a singer/songwriter, and Edge, as the main composer/arranger and key instrumentalist, would be able to continue making commercially accessible music without the other guys. Bono is so famous now that any solo disc he released would be a guaranteed hit. Edge would probably lend himself out to various projects, occasionally playing shows that he headlined himself, record soundtracks, etc. I do not see Edge at 50 joining a full-time supergroup like Velvet Revolver... but I could be wrong.
Adam and Larry, as the rhythm section, would be hard-pressed, not to join another band --- which they obviously could --- but to be taken seriously. Whatever they would go on to do -- as Adam has previously pointed out -- they would forever be thought of as "the bass player/drummer from U2."
Maybe it's more interesting to speculate what would have happened had Edge actually quit the band back in 1990 or so. The other guys had so much more to give, I find it hard to believe they would have packed it in barely out of their 20s. Would they have kept going, like R.E.M.?