[nbadg]

I don't personally want to make a value judgment about whether or not you could call supersonic air travel sustainable, particularly since it's fairly meaningless to remark on outside of a broader conversation (in much the same way that the "greenness" of electric cars is more complicated than saying they're zero-emissions, since you need to consider the power plants that are ultimately fueling them, plus the batteries, motors, manufacturing...).

But I do want to note that supersonic aerodynamics are extremely complicated, and I think it's just as disingenuous to say that drag always increases with the velocity squared in supersonic regimes. Strictly speaking you're right, but the drag coefficient itself is changing too, and in fact it converges to a value below that of its approximately-constant subsonic value. Also note that airlines are already traveling thoroughly within the transonic region.

I do agree that ultimately, in-atmosphere supersonic flight will consume more fuel than commercial airlines today. However, to say anything more specific than that, I think you really need to start doing a proper aero analysis; once you're transonic (or beyond), you've stepped outside the realm where napkin math is physically valid.

For a brief, okay discussion of drag coefficient changes wrt mach number, here's a link: https://onlinelibrary.wiley.com/doi/pdf/10.1002/978047011785...