Comparing chess players from different eras is futile. The play has evolved as the game's mysteries continue to be revealed with the use of computers and those solutions widely disseminated. Fischer's opponents were lucky he quit when he did because he would have dominated for years. But the player who dominated his era more than anyone else is Paul Morphy.
It used to be futile, but now with the advent of computers, we can check how error free, or how complex, the positions of past and present players past had to deal with.
Fischer found no problem comparing players from different eras, and he blatanly said that if Morphy was alive in his (Fischer's) day, he would beat everyone. I agree with him that it is very possible to compare players from different era.
There is no question that if Fischer played with his knowledge against a modern top player with their knowledge, Fischer would likely be the underdog. But, if you gave the
young Fischer 2 years to study the game today, in two years time Carlsen or Kasparov, __anybody__ would be in grave danger of not being the best player in the world against a prime modern Fischer. Would that be true of Capablanca? Or Lasker, or Alekhine? I don't know, but I have zero objections of saying this about Fischer.
Carlsen has to be about 2925, and dominate the best players in the world for five years, before I consider him the best player ever. In my mind he is not yet better than even Kasparov, who even at 50 if he came out of retirement could school Carlsen. On the other hand, I know of no one who has written more masterly chess books than Kasparov.
What will Carlsen's "My 60 memorable games" look like?