Good post phil star. Murray just isn't as quite out and out aggressive as Fed (but who is) or even Nadal/Djokovic. He is becoming more aggressive though - that has come through clearly from Lendl.
Also, once you have Fed and Nadal hanging up their racquets - they'll go long before Murray is done IMO, then who's going to be winning the slams? I don't see any major competition outside Djokovic.
Think of this...until Murray and Djokovic are 30 there are 20 slams or so to be played yet...who's going to win those? If Fed wins no more - then you've got a lot of slams to go around 3 guys...