BA, one of the two Concorde operators, started using lie flat beds in 1999. Six hours vs three in a Concorde for JFK-LHR is totally a worthwhile trade for a cheaper fare if the six hours are mostly during sleeping hours anyways.

There were definitely other problems (the aircraft was a dinosaur, with parts no longer made and a three-person crew; there was no reasonable replacement program on the horizon; the fuel costs of supersonic were hard to justify with the oil prices of the 2000s) but yeah, at some point Concorde's proposition became less and less appealing.

FWIW the biggest supersonic contender these days, Boom, is specifically targeting room and per-mile costs for a modern business-class cabin of 50 people, which is a lot more workable from a business perspective. That being said, the money is not really in transatlantic (where you can sleep for 6-8 hours anyways) but in transpacific, because 12+ hours in a plane is a slog no matter how comfy the seat is.