I'd guess that his ceiling is Bradley Wiggins. He could get as skinny as Wiggins and beat the GC guys in TTs, but he still wouldn't be quite a good enough climber to win a grand tour. Is it worth giving up being the best TTer in the world for that? I don't think he's dominated the TTs long enough to be bored with it yet.
Not only that, but you have to wonder if the trend of increasing TT kms is going to last. 2013 will be the 100th TDF, so I'm sure they'll have tons of climbing there. They did make the Giro "easier," but it still favors the climbers and I don't think that will ever change.