There are a bunch of cocky people on each side, and yes nothing is guaranteed. I agree with you on that one. The perfect example is McEnroe's incredible 84 season, followed by never winning a slam again. I do however think we need to stop all this speculation, and as MN has stated, the situation is completely different between Federer and Nadal at the moment. Nadal is very much top two, holder of two slams, and a slam final and with many more years to play provding he is still fit and healthy, and motivated. Federer can strike gold, but realistically his chances are less than Nadal, I think that is a fair assessment. It's doesn't mean Federer won't win, because he was only two games from slam 18, just means Rafa is more likely to add slams than Roger is at this moment.
The real question is has the Undertaker wrestled his last match, and what the hell is going to happen with Brock as champion?