He probably won't reach #1 again.. Though he should shoot for it since hes so close to passing Sampras in total weeks but realistically he probably won't get it back. If he can manage 2 slams before he retires that will probably be enough to keep his slam count safe. If he stays at 16 that may not be safe.
Why not? Djokovic won't play like this forever, he is a HC monkey. Federer is mental, he was competitive in several matches with Djokovic but choked when Nole didn't just roll over and die. If he were to get his act together and play to the best of his abilities (much like Djokovic is now) he can easily win slams. Forgotten how convincingly he got WTF? US Open and Wimbledon are not much slower than that.
And I wouldn't expect Roger to fall apart again to Rafa. I mean it certainly could happen, but remember 2008? He got trashed at RG and saved Championship points against him in a five setter at SW19 and could've won that.
At USO, he managed to achieve a 5 setter and holding MPs against the runner up while serving like 35% 1st serves.
What I'm saying is, he is physically capable of doing everything you claim he isn't. Whether he WILL do them however, is a different question.