How exactly? He has a losing record to all of the Big 4 and has lost to Roger a number of times this year alone.
Because he's young and 2 of the big four are likely to be gone while he's still playing.
Federer will retire in the next few years, and Nadals knees will catch up to him. Djokovic and Murray will be past their prime years while Del Potro is still in his prime years. There also doesn't seem to be any 90's players looking like they break will break through in the next couple of years