I think the reasons for REM splitting (which seem so organic and almost idyllic - so graceful) will be different from the reasons why U2 will eventually split.
I'd be more inclined to think that U2 won't ever officially split, but just fade away until special circumstances dictate that they really are over. Bono might fall into a role or a job that demands the need to temporarily retire, but the possibility of at some stage picking up where they left off will always exist.
In no way this will happen
U2, and specially Bono will make an event out of it, to make a media fest and sell yet more records.
Then the inevitable promotion interviews, U2 Greatest Hits albums, another remaster of studio albums ...