Federer is a champion. He's had a tough stretch lately and is coming bac from an injury, but in this year's Australian Open, he seems to have rediscovered his serve. Also, Murray's exit helps Federer with his draw. Technically he's still one of the best in the men's game and recently beat Nishikori.
His recent streak and improved performance clearly show he can. Of course, there will come a time when either age, injury, or illness prevents him from playing at the top of his game but I don't think it will be anytime soon. I am certain he didn't train up just to let it all go.
Still, no matter how marvelous Federer looks in his Grand Slam defeats to Djokovic, the losses point to the obvious: Djokovic is the superior player now and will be for the remainder of Federer's career. But when you've been the best, how difficult must it be to stand there and watch others give victory speeches? How pumped can you get for yet another title in Cincinnati, Miami, Dubai or any cities other than London, Paris, Melbourne or New York?
Comebacks are just too hard to do. As soon as a person stops training seriously, their ability to do their sport at the highest level goes away. The number of people who have successful made comebacks in sport are very few. Aly Raisman is a rare example of a successful comeback in gymnastics. Federer can't do it.