With great respect for NM Reb, a comparison of longevity between Lasker and Kasparov is misplaced. Lasker did not have to defend his title every three years and he did not always have to face the best challenger as selected by candidates' matches. The conditions of the longevity matter. Kasparov's reign is unparallelled both for the amount of chess and for the quality of opposition he had to play in order to continue it. And the grueling matches with Karpov were truly epic; Lasker never had to face anything like that.
I tend to agree with maestro Reb, however, about Fischer's strength in his prime. There were enough players who overlapped Fischer's and Kasparov's careers that a worthwhile comparison can be made based on common opponents.
Capablanca and Alekhine played 50 games between them, and we have a bunch of games that Lasker played with them (mostly Capa) as well, but not even one Fischer-Karpov game.
It doesn't seem likely that Fischer could have held it together and been stable enough to play Karpov for many years, and then Kasparov, but if he had we'd have been enjoyng our own modern "Big 3" era.
No such luck though.