That these planes were operated only by BA and Air France (and a similar plane to some extent by Aeroflot) was merely an accident. Concorde was developed as a mass transport and there was broad interest. Then the US ban and the oil crisis happened.
The reach of aeroplane travel is far more impressive now than flying under a 100 people at supersonic speeds.
I also dispute that there was mass transport plans - the total passenger load was tiny.
Not to take away that the plane itself is an amazing piece of technology.
A Concorde B variant with an even longer range and quieter engines was already being developed, and airlines were very interested in it. if the US market had been more open it's likely other markets would have followed suit.
Once it was stabilised in the market other improvements would have followed. Concorde was always a first class for the first class market, not an air bus. But if supersonic travel had become established there would have been market pressure to commoditise it, and we might have seen a continuing supersonic long haul market working in parallel with the smaller more local subsonic services we have now.
I dispute that airlines were actually interested in a Concorde B. The economic problems of supersonic flight would have remained.
There's a reason Boeing abandoned any pretense of interest in SST after Congress declined to fund it.
I do think supersonic was just priced out of the market. Would you pay twice for the time saving?