That's been said for how many years now? In fact, he may actually have more of a chance as the miles start to catch up with the likes of Nadal, Djokovic and Murray, who now are no longer 23/24 year olds, but around 27/28.
The HC slams will be too difficult with Nadal and Djokovic IMO, but if he keeps his fitness up and keeps this serve volley routine going, I think Wimbledon next year will be a very good chance. Still, quite comfortably the best mover on grass.
I kind of agree with what you're saying. Nadal and Djokovic have already started to decline, and with the nature of their games, their decline (IMO) will be quicker then Federer's. There is another final in him for sure. Its perilous to write off a guy like him, may even go into beast mode and pull out a HC slam.