Fed has been in decline ever since 2008. I know there are certain fans having come up with whatever excuses you can think of - you name it, mono (yes it had its influence, but just don't exaggerate it), "he doesn't care anymore", etc.
Fact is, no player stays on top forever, and Fed has already done an awesome job when it comes to consistency. He's dropped a few paces by now, such should be obvious. But as long as he remains in the top #4, he'll remain a contender for yet another slam title. Never count out a great champion like he is.
That said, even if he will remain at 16 GS titles, Fed fans will have nothing to complain about. I am just very happy that his decline seems to be a very gradual one, and am hoping for some late brilliancy from him in what might well be his last season.