I think at one time, when they were a real band, that U2 would never dream of going on as U2 if one of them left. But I'm not so sure about that anyone. Even as far back as Zoo TV, in his book Flanagan writes about how the other three were fully prepared to continue on without Adam.
IMO, IF Larry were to leave and IF Bono, Edge and Adam still wanted to record as U2, they would. I don't think Larry leaving, alone, would case the others to quit. Though as I said, I think it's also quite possible that they won't be making any more full "records" at all after this one.
But of course, this is all just supposition on my part. There's only four people in the world (well actually two) who can say any of this definitively.