Steinitz didn't do as well against common opponents with Morphy, and he got destroyed by Lasker. And didn't he also have an overall minus against Tarrasch who was busy being a physician?
Lasker didn't defend his title for long periods of time and got destroyed by Capa.
Capablanca as number 1 can be challenged by his loss to Alekhine.
Alekhine can be challenged by his loss to Euwe and his ducking Capablanca for a rematch.
Botvinnik had nearly even records in WC wins Bronstein, Smyslov, and Tal, and he had the benefit of the Soviet collusions against Reshevsky and Fischer.
Fisher as number 1 can be challenged by his refusal to defend his title.
Kasparov as number 1 can be challenged by his loss to Kramnik, and that he barely eked out victory (though in 3 matches nonetheless, amazingly so) against Karpov, who himself struggled in one of his WC matches against an older Korchnoi.
Morphy, though, has fewer of these direct challenges to his dominance ... People cannot challenge his chess accomplishments directly, they can only challenge the quality of the masters that he hardly tried against. The other thing about the other champions is that, with the exception of maybe Capa, none of them just suddenly burst onto the scene and dominated in such a manner.
I agree that a freshly time-transported unbooked up Morphy may struggle early in an all-time match-up, but genius is genius... Everyone in the mid-19th century had whatever similar resources they had, but Morphy dominated without even the tools those players of the mid-19th century had, namely a stream of the strongest players to play regularly (being in the US instead of Europe). I agree with Bronstein and Fischer, that it wouldn't take much for Morphy to catch up to modern GM's and then I go a step-further to say Morphy would surpass them ... I understand the last part is an opinion, but that is what this whole thread is, arguing over opinions.
I also think it is unfair that these greats are being compared to computers ... The romantics liked to gambit and it was considers unmanly to back down. They could calculate variations as well as modern day GM's. If you destroy your opponents so easily, you don't play your best or most accurate/computer-like. If you are up 5-0 in football (soccer in US), you might decide to rest your star players for the last 20 minutes of the game so they can be refreshed for the World Cup, rather than run up the score to 6 or 7 goals to further prove your team's greatness. Or if your team is already advancing, you might rest your star players in a game that doesn't affect the outcome. It would be unfair for future generations to say "oh they only won 5-1, but the other team 50 years later (who kept their star players in to break a record) beat a more modern better team 6-0, so they are better."
And as far as the advantage of modern knowledge... There are 1800 players who know more about openings, pawn structures, and modern advances than Morphy, but none would come close to beating Morphy. Modern advances are knowledge-based, not logic/computational-based.
"Here is an opinion from a player who was stronger than all but a handful of the members of chess.com, who drew a match for the World Championship with Botvinnik, played against World champions Fischer, Spassky, Petrosian, Smyslov and Tal and couple of hundred more of the strongest players of the 20th century, played in abouty 300 tournaments and matches in his life (compare that to Lasker with fewer than 50) and wrote one of the best chess books ever (Zurich 1953 Candidates)"
He was still wrong though.
A blunt disagreement is not going to fly. You will have to tell us why you think he was wrong if you expect us to accept your opinion over that of someone that probably knew more about chess than all the contributors to this thread put together.