Also physics. Supersonic travel is prohibitively expensive because it requires exponentially more energy than flying at ~900 km/h. More efficient engines can't really change that much..
I wouldn't mind a Gothic cathedral if I could get one for cheap and maintenance didn't cost anything.
At the end Concorde failed because it was mainly a status symbol (for the governments, airlines and passengers). As long as subsonic remained considerably cheaper (and it probably would because of physics) I'm convinced most people would still ussually choose it even if supersonic became more affordable.
I mean it's like saying most people would prefer to fly first class if it was more affordable, well.. obviously.
