The desire to have some kind of "friends" is surely why Owl put up with Megg and company for so long but I think the end of Megahex is exactly him deciding that being "alone" is better than being with those horrible people.
Dramatically, it would be a bad idea for Owl to ever show up again, I think. The theme of that story were he leaves is "escape".
And for Megg this is the first sudden fall-off in her slowly deflating life. No longer loving Mogg seems to be next and WWJ's suicide should come soon. Then she's visit her mother.