Well he did something that few players have been able to do. He adjusted to not being in his peak 22-27 year old form, and has been able to maintain a very high level since. My sense is that most top players can't adjust, or don't want to be 90% of what they once were.
We're also seeing no signs of further decline. I mean, he's just as good as he was last year, with no signs of "decay" in his game. I don't see why he would just fall through the floor next year. I think it will be gradual. I mean, he's been pretty steady since about 2010 to the present. Its also worth nothing that while he isn't as much of a threat at Slams as he was in 2012 and before, his winning percentage has actually been better in 2012, 2014 and 2015 (86%) than it was any year since 2007.
Also, there's the fact of the field. Just as Novak will benefit from entering his age 28-32ish phase with a weak younger generation, so too will Federer.