Is there a mileage bubble and, if there is, what will pop it? @:-)
I don't think that there is. Miles can be on the books, but I don't know that they're an issue unless they're cashed in. And if awards are always limited, I don't think that it'll get out of control.
Now if saver awards were available like standard awards (meaning any available seat), I think there might be more of an issue.