At the risk of partisanship, I still don't think that Djokovic is mentally the equivalent of Roger and Rafa. The post-RG sink has been surprising. And no one believes that it's 100% about injury. Both Federer and Nadal have demonstrated bounce-back potential over the course of their careers that we have yet to see from Novak. Basically, Djokovic rose to the heights, and hasn't really had to deal with set-back, until now. Roger and Rafa both still seem very motivated, and have each other as touch-stones. In terms of motivation, I doubt that this year would cool them off, but rather incentivize them. It's going to be age and health that would put them off. Roger's the old man left at the top, and he's got an excellent resume. If anyone should lose motivation, it would be Fed, though by everyone's account, he loves tennis and wants to keep playing. Rafa would be more motivated to catch him, and to stay ahead of Novak, if the body holds out, he's only 31. Novak is the wildcard. I get why you pick him for YE#1 next year: who else? Fedal have a lot to defend, and hopefully they're going to be judicious as to when they play.
As to your last, I think you could be right to say that when Roger hangs them up, Rafa might not be far behind. That will most likely depend on his knees, and how much Djokovic is breathing down his neck. But, hey, if they're still winning Majors, or close to it...