I think the interview snippet, along with all his hints that he has achieved his objectives on its own highlights what is his most likely motive.
He clearly doesn't think of SAO as 'just a game (he developed).'
Maybe there's some social experimental motive underlined behind his crazy 10,000 hostage scenario, but it feels like he's just a game developer who's just that invested into SAO that he wants it to be 'a real world.'
Then again, the flaw with this theory is that if he's going so far as to remove the log-out button... why not make it even more realistic by removing the menu function altogether? If Kayaba was really into full-fledged fantasy role-play, the concept of obvious game features remaining ( messaging,menu, etc) makes it less 'real.'