Petrosian is one of the most important players in chess history. Any fan of Karpov or Nimzo should also be a fan of Petrosian. Karpov took Petrosian's style and added an incredible will to win instead of draw. Petrosian all but invented a whole array of exchange sacs which took Nimzo's ideas to a new level.
Petrosian's games are truly amazing. I have Shekhtman's two volume collection of Petrosian's games and still go through them occasionally.
I was going to give some examples, but then I ran across this article by a young GM which illustrates my point even better. Today's young players also study Petrosian!
http://www.chess.com/article/view/petrosians-exchange-sacrifices-explained
You must take into account that a player at any time, if they play, can keep up or even better the competition... from Morphy to Fisher to Kasparov to Carlsen. A GM or a World Champion can move forward from any particular point of knowledge. We all can learn even at 90! I'm 71 and I play chess six hours a day and I still learn. Today's puzzle is a great example of the power of a Knight postioned properly.