Just havnt seen where his game is complete enough to win a a slam. A big serve is expected these days, not that it will win you slam. Got a lot of work to do if it's gonna happen. Seems fairly unlikely.
Movement and footwork > huge serve. The current top 3 are impeccable movers, that's why they hold 33 slams between 3 of them. Serve is only a minor part of Nadal's and Djokovic's games.