I predict about under 3000, but not more. The bigger the gap, the less points he gets with each win. Even draws will make him lose points, almost like a loss to a "normal" player. This will limit his rating.
There is a new pack of youngsters who are showing great promise at an even earlier age, so Carlsen may well face his own young challengers quite soon.
what is magnus rating peak going to be when his career is over?