Fed still has another 2 years at the top IMO and Nadal is only a year older than Novak. Nadal made it to many finals this year and the only person that could stop him was Djokovic so he will still be around as long as his knees hold up. Unfortunately Andy Murray has no forehand or mental fortitude so I will count him out. Its not as bad as you are making out though.
Federer is now 30. You can count on the fingers of one hand the slams won by men in their 30s. It is possible but unlikely that he will win another but he certainly won't be number one again.
Djokovic is clearly the boss now. The key was dealing with his coeliac problem. Since then he has been supreme. He is younger than Federer, a better player with more weapons than Nadal and clearly ahead of the rest. It is conceivable that Nadal might trouble him on slow clay but it unlikely that he will beat a fit Djokovic on any other surface.
I think Nole should be credited with his achievements. Had his coeliac condition been diagnosed earlier it is highly likely he would have taken over at the top long before he did given Federer's flakiness, Nadal's consistent inability to beat him post diagnosis and the inability of the rest to compete at the business end of the biggies.


