The key point is that the seven aircraft (two of which they paid £1 for!) spent very few hours per week in the air, because whilst it was profitable on one transatlantic route at very high prices, it would have lost money on just about any other route or with more frequent operation. And to have a profitable airframe programme you need your customers to be able to operate more than a couple of routes.

(the 40% figure is more an indication of BA's sometimes thin margins than massive unfulfilled potential)

My understanding is that it was profitable if you got the aircraft for free and already had pilots capable of flying them.

If the Concorde had been an actual financial success they would have developed it further and made a successor. And if BA and Air France had thought that the Concorde would continue making them money they wouldn't have retired it after one tragic accident in 30 years of operation. The 737 Max is still being made after much worse.

If I recall correctly, that's one of the well documented places where I had understood the fact that the concorde wasn't profitable, and would more or less never by in its current context: https://www.youtube.com/watch?v=sFBvPue70l8

Which isn't to say Boom may not succeed.

First class NY to Paris in a couple of weeks, one way, is ~$6k "best overall", with $10k for "fastest" (checked a random flight on Skyscanner).

$10k transatlantic return hop. Add expedited immigration. Does not sound insane. First class tickets London-NY can be already in this price range. Not for everyone of course (certainly way out of my price range).

I'd expect for trans-oceanic, you'd have people scheduling their travel around limited flights, given the unique offering.

I.e. you aren't trying to figure out "How do I 100% capacity a 8:17am daily flight?" (traditional subsonic carriers) but rather "How much demand is there per week/month?" (Boom)

If the flight is Wednesdays-only, then folks line their travel up on Wednesday. Because the alternative is a much longer flight.