Hmm, yeah. I mean with modern technology it should be feasible to create a more cost-effective / greener supersonic airliner than Concorde.

But basic physics dictates that supersonic flight requires a lot of power, as well as low bypass turbofans. So competitive with modern "normal" airliners in terms of cost or emissions per seat-km, nope, not gonna happen.

Synthetic jet fuel is a good idea that deserves R&D money (if we're gonna keep flying long distance in a carbon constrained world I think something like that is going to be necessary), but is orthogonal to a supersonic airliner. Unsure why they think that bundling synthetic jet fuel (itself a high-risk R&D project) with their supersonic jet (another high risk R&D project) will do anything but increase the risk of failure of the entire project. Well, the uncharitable explanation why they're doing it is of course greenwashing.

Concorde had a cruising speed that was 2.31 times higher than Boeing 747, and a lift-to-drag ratio that was almost the same times lower (2.36). So, every second Concorde had to burn about 2.36 times more fuel per ton of aircraft compared to 747, but it reaches the destination 2.31 times faster. All in all, the fuel consumption during the cruising phase of the flight was comparable. Well, Concorde had a horrible fuel economy during takeoff, so overall it was pretty bad compared with the 747.

But 50 years have passed since the Concorde was designed. Both computational fluid dynamics algorithms and computing power have made tremendous progress. It's quite likely Ouverture will have a much better fuel economy than Concorde, if only for the fact that it will use turbofan rather than turbojet.

In theory, a faster airplane is more productive (fly passengers farther in the same amount of time). So crew and capex costs are lower per mile. Maybe a crew can fly New York-London and then fly back, vs a subsonic airliner with a crew that flies one way, stays a day for crew rest, then flies back.

Cheaper than the current options for transporting 80 people via supersonic aircraft.