Originally Posted by Dynasty
Kasparov's primary legacy is being World Champion for fifteen years.
Ratings achievments just aren't that big a deal.
I've read several of Kasparov's books, and I never quite got a handle on his personality in the same way I get Fischer's, but at no point has Kasparov ever struck me as a man who doesn't care about little things like that. In fact, he has always come off to me as a hyper-competitive person who fought and clawed for every possible measure of success. This was a guy who made no bones about accusing people of cheating if they gave him trouble in meaningless exhibition games...including a 17-year-old girl. (that's not to say they WEREN'T cheating, just that's a glimpse into his refusal to allow something to cast any doubt in any measurement of his chess ability)
It's like Gelfand said, Kasparov doesn't miss any opportunity to remind everyone that he used to be the best chess player alive.
That being said, the 2851 is a super-impressive achievement of his. It being surpassed I would imagine would be seen as a major event and would draw quite a few headlines, and I'm guessing that it's the exact type of thing that would cause Kasparov to make a "defensive move" in order to defend his stature. Probably something subtle, as has been said, but something unmistakeable.