Presumably, you have to be travelling quite a distance before hypersonic/high-altitude flight becomes more efficient than current commercial airliners.

Certainly not for short hops, but if regular aviation is any indication, it’s more efficient even if you don’t even reach cruising altitude before beginning to descend, so I’d imagine any flight that currently is around 4 hours or more would potentially benefit. (Given full technology maturity) that said, we are a long, long way from having efficient engines for that wide of a speed envelope.

What about noise? So far all we have gotten is companies trying to build planes that still are very loud and produce audible booms all the way along their travel path. Pitched to the empathetic heros that are billionaires. And to sway public opinion they fly test planes at slightly over mach one in specific weather and altitude setting that avoids having the sonic booms reach the ground. While marketing and selling the idea of not mach 1.1 but mach 2-3 supersonics. Which is AFAIK completely impossible without the boom issue, even if made less bad - but arguably still bad - by fancy wing shapes. So all I see is selfish people trying to build something at expense of everyone else, including the environment.

Flying it the high limits of the atmosphere reduces sonic boom intensity due to both the extremely low density of the air and the distance from the ground. Ultimately, sonic boom is a sign of wasted energy, so if (big if) we can make aircraft that conform to the required aerodynamic ideals for efficient hypersonic flight, they will be relatively quiet.

Just like with regular air travel, though, I would expect a few decades of practical application for specialized service before it was commodified to be really useful for large amounts of passengers.

what about noise is you're supersonic noise and sound is about a mile back

I think the concern is for the frothing masses of earthbound unfortunates, not the passengers hurtling gloriously towards destiny miles above the troublesome surface of the planet.

> the fuel cost of getting to altitude is mostly returned in the descent

In energetic terms this is clearly true, but that leaves you at the end of the descent with very high speed / lots of kinetic energy. Great for missiles, because you dump the energy into the target. But I'm struggling to see how it's useful for any other application?

I would assume that any efficient hypersonic airliner would need to use variable geometry or powered manipulation of airflows to attain a reasonable efficiency in the transition zones of its flight envelope, so while you obviously would not get all of the acceleration energy back, it would transition into efficient lower velocity flight and reduction in engine thrust needed during the deceleration phase. But if we really knew how to do all of this we would already be flying transatlantic in an hour.

For the same reasons that theoretically possible doesn’t mean necessarily practical, theoretically difficult doesn’t mean practically impossible. We just won’t know until we significantly advance the state of the art. We might find out that atmospheric flight is a waste of time and that exoatmopheric ballistic trajectories are where it’s at. Or we might find out that zeppelins were the way all along.

We are still a long way off from having engines that can operate any where near theoretical efficiencies except within very narrow parameters and altitudes. Until we sort that we will have difficulty traversing a wide speed envelope without waste.