> If the people currently flying business class all move to SSTs

Which is vanishingly unlikely for almost all routes. A proper premium F or J experience that takes a few more hours is going to be a better deal for most people than a Y-like experience that shaves some hours off, like Concorde was. For a similar parallel, look at luxury travel. Private for the short hops, but the premium cabins on premium carriers are the better experience for longer journeys unless you're one of the truly tiny number of people flying a G6.