I get the impression that part of MMM's motivation for blogging is to spread the good word. The side income is a nice bonus, but not the only consideration.
With that in mind, selling it wouldn't really scratch the right itch, would it? If there's an evangelical bent that motivates the author, then cashing out and disappearing just means losing the flock.
Cashing out and staying on as an employee would fix that, but I'm not sure he's keen to be an employee again. I think that he's like so many other people, willing to work hard on things that are fun and optional, less willing to work hard on things that are mandated by someone telling you what to do.
Now having said that, everyone has a price. I'd have to be damned in love with a project to turn down a billion dollars, if I thought that money could do more good towards my real goals than my continued personal participation.