You are overlooking the obvious problem. Subsonic planes already existed. Even it it was just as efficient at subsonic speed, I doubt if it was as cost effecti

You are overlooking the obvious problem. Subsonic planes already existed. Even it it was just as efficient at subsonic speed, I doubt if it was as cost effective as a 737, for example. The Concorde seated 100 people, in first class. If you can’t fly at supersonic speeds, you aren’t going to spend the extra money.