Trying to predict the future of a great player is tremendously difficult. Someone here already mentioned the case of John McEnroe not winning a major after 1984. Nobody saw that coming in January of 1985.
Here's an even better one: imagine for a moment that you (the 2009 you) have magically been transported back to the summer of 1980. It is early August. Bjorn Borg has just won Wimbledon a few weeks ago for the fifth straight time, beating McEnroe in an epic five-set match. This is the tenth major of his career (five French Opens, five Wimbledons). He is 24 years old, and playing the greatest tennis of his life, seeming to get better with each passing year since his career took off in 1974. If he wins the upcoming US Open, as seems likely, he will take the unusual (for him) step of going down to Australia in December to try to complete the Grand Slam.
He is being called by some the man who will go down in tennis history as the greatest player of all time, no questions asked. You offer knowledgeable tennis people (writers, commentators, other top players) a choice of two propositions:
1) Bjorn Borg will win ten more majors in his career.
2) Bjorn Borg will win one more major in his career.
I think the majority of those knowledgeable people, the OVERWHELMING majority, dare I say, would pick number one. The idea at that point the Borg would win only one more major would have probably seemed slightly ludicrous. Yet we here all know the real history.
Predicting the future is hard.