Imagine a young Federer, chasing his first Wimbledon title, meets the current Djokovic or Murray at some point in the tournament - there's a fairly good chance he gets knocked out..
Now lets say he keeps bumping into those two, Nadal also, during the latter stages of slams over the next year or so..
Now he will won some, but he will also lose some.
What you'll have then is instead of a brilliant young player, full of confidence and a few slams to his name, steamrolling through the competition (like what actually happened) - You'll have a guy who reaches 24/25 with maybe only 1 or 2 slams, at most.. hovering around the top 5 in the rankings.. wondering if he'll ever be able to dominate like that Novak guy at the top.
My point being, is the guys consistently residing in the top 10 are not weak players, but potentially great players with the misfortune to play in an era where there are still 4 players on tour who are virtually unbeatable if on top form. They can't build momentum.
Also, these 4 players are arguably the fittest in tennis history, with no technical weaknesses and comfortable on all surfaces. Their longevity at the top is all to do with them, not the inability of everyone else.