But he basically only managed it for 2 years. And then gave up. I think his lack of sustainability should put his achievements in a more suitable light.
You are only looking at Fischer last 2 or 3 years. After he won his first US Championship (1957) at age 14 he won EVERY event he played in in the US except 2 ! One of them was a second to Spassky in one and I forget the other. Many believe Fischer was already the best player in the world by age 20 but he didnt play a match for the title for another 9 years due to various "problems" some of which were of his own making, true. Fischer was trying to bring more money and better conditions for the top chess players and he managed to do that. Recall that when Spassky won the WC against Petrosian in 1969 Spassky got a sum of like $2000. !! Thanks to Fischer they now play such matches for hundreds of thousands and even winners of Open tournaments get more than Spassky got for winning the world championship.
Off the main topic--longevity and best are two different sets of criteria. At his peak during various periods both Karpov and Fischer were unbeatable. Kasparov was not far behind. I think all three of these players played competition (at the time) the best in the World--rankings can be tricky, for instance Bareev was once rated #4 in rhe World without ever having won a major tournament. Fischer record up to his 1972 match is a streak that has not (and probably never will) be matched. Also Fischer tended to work alone with less resources than the other two (K and K)--personally I think he was the best ever (as a chess player)
Another interesting parallel is Fischer and Michael Jackson--both geniuses in the field but messed up in their personal life--much better comparison than the Fischer/Hitler one about 75 messages ago.