Building an entire airline around it is stupid.

Building certain premium flights around it, not necessarily; Singapore Airlines operates a 67-seat business class on a plane from Newark to Singapore that only has business and premium economy. BA operates a 32-seat business-only flight from London City to New York via Shannon. And these are just the subsonic flights.

JAL is one of the bigger investors in this project, so it's not as if there's no interest at all.

They’re almost always money-losing ego flights.

BA1 rarely flies full. The ego is right there in the number. It’s a novelty.

SQ hasn’t operated business class only since like 2011. It has 67 business seats and 94 premium economy seats. And still probably loses money.

The one you didn’t mention was Stavenger to Houston which... only flies when oil is flying high. Because it’s not profitable otherwise.

Even so, the main point of my comment was that the maximum capacity of a Boom supersonic plane is a lot smaller than that of the Concorde (and presumably a more comfortable layout rather than the Concorde's narrow cabin.) So at least the battle it's fighting is a lot less severe.

Theoretically speaking, if the seat cost per mile is as low as advertised some airline might want to take some business class seats out and put some premium economy in. What a plane does with seating layouts is its own concern.

You don't need 'certain' premium flights to make an airframe programme commercially viable though, you need 100+ which airlines are sufficiently confident of to make downpayments on aircraft, and it's not like such routes haven't been unsuccessfully experimented with before.

As they say, the best way to make a million dollars is have a billion and start an airline.

David Neeleman might disagree. But then again, his M.O. isn't chasing business customers.