We don't know what would have happened with an active Morphy during the two (or more) decades after 1857-8 because, for the most part, Morphy chose not to be involved with the serious chess world during the two (or more) decades after 1857-8. We are able to consider decades of post-1873 serious Steinitz chess activity.
"... Generally considered to be the world's strongest player from around 1870 to the early 1890s, Steinitz was ..." - IM Craig Pritchett (2011)
"... It was due to [Morphy's] principles of development that he had, in most cases, at the outset a better development than his opponent. As soon, however, as these principles of Morphy's had become the common property of all chess players it was difficult to wrest an advantage in an open game. ... the next problem with which players were confronted ... was to discover principles upon which close positions could be dealt with. To have discovered such principles, deeper and more numerous as they were than those relating to development in open positions, is due to Steinitz. ..." - Richard Reti (1923)
"... the next problem with which players were confronted ... was to discover principles upon which close positions could be dealt with. To have discovered such principles, deeper and more numerous as they were than those relating to development in open positions, is due to Steinitz. ..." - Richard Reti (1923)
The Steinitz-Zukertort world championship match was 16 years after 1870.
"... Steinitz ... started out as an all-out attacking player, as it was common at the time, but then went on to change his approach toward chess and became very positional. Positional, at that time, was very shocking to the rest of the chessplayers, and they actually considered his new way - his new style of playing as cowardly and controversial. It was only, later, his successor on the world champion's throne, Emanuel Lasker, who acknowledged the influence and the impact of the concepts Steinitz introduced. ..." - IM Anna Rudolf (2018)
https://www.chess.com/video/player/games-that-changed-chess-history-part-4
https://www.chess.com/blog/janwerle/finishing-touch-from-the-world-champions-2
https://www.chess.com/article/view/behold-steinitz-the-austrian-morphy
https://www.chess.com/article/view/steinitz-changes-the-chess-world
https://www.chess.com/article/view/steinitz-the-official-world-chess-champion
… "Edge wrote in a dispatch to the 'NY Herald,' Jan. 5, 1859 :'Paul Morphy had declared that he will play no more matches with anyone unless accepting Pawn and move from him.'"
An unwillingness to play a match of level games is not a challenge to the world to play at odds.
… If Morphy had made a real challenge-to-the-world without financial provisions to compensate someone taking up the challenge, then it seems to me that Morphy would have been guilty of making an empty gesture. Travel cost alone could have been a substantial obsticle for someone thinking of taking up a challenge-to-the-world without the potential to receive money. On the other hand, if there had been a real challenge-to-the-world with a potential for substantial monetary gain, it would be of some historical note if nobody took up the challenge-to-the-world. If there was no real challenge-to-the-world, then there was not an empty-gesture-challenge-to-the-word and there was nothing noteworthy about an absence of takers for a challenge-to-the-world that never took place. ...
I suspect that that this obituary is the origin of the somewhat mistaken perception that Morphy ever issued a challenge to the world to play a match at odds:
https://www.chess.com/article/view/on-the-death-of-morphy
Btw, "king" and "queen" of Spain are visiting New Orleans, you can have them forever if you ask me.