Another factor in this mix is frequency, which matters a lot, especially to business travellers.

A once-daily supersonic flight might minimize “time in the air” while a once hourly mostly-economy 737 shuttle minimises “time away from home.”

There is one airline that flies all business class. An A321Neo with 76 lie-flat seats, NYC to Paris/Nice/Milan. Random date selection yields $2700 one-way New York to Paris.

https://www.lacompagnie.com/en/about/services

British Airways operated a similar flight between 2009 and 2020: an A318 between LON and NYC with 32 lie-flat seats

https://en.wikipedia.org/wiki/Club_World_London_City

Smaller jet aircraft on the same route generally means relatively expensive operating costs: some costs like landing slots and pilots are essentially fixed, whilst others like maintenance, fuel and capital costs don't scale down linearly. The marginal profit on an individual economy class seat might be small, but 100+ of them cover a large portion of the fixed and semi-fixed costs of operating the aircraft, and are relatively easy to fill.

Long range business jets which can comfortably accommodate a typical narrowbody business class cabin exist: nobody is certifying them for all-business class scheduled flights because it wouldn't be profitable to do so; likewise the all-premium 32 seat A318 configuration hasn't been adopted anywhere except the NYC/LON route it didn't really have the range for because it wouldn't be profitable elsewhere. Boom's bet is that supersonic changes that.

> some costs like landing slots

Small detail: most landing slot costs are variable based on aircraft weight.

Landing charges levied on each landing usually have a significant weight component (amongst other variable components like emissions, noise, handling and passenger charges) but the relationship usually isn't linear with passenger capacity. Landing slots required at busier airports to have the right to land at a certain time each week are generally traded between airlines with the slot coordinator's agreement with the value of the slot based mainly on the commercial attractiveness of the time slot.

Absolutely, good point. I don't know why I latched onto charges vs slots.