The faster you go in an atmosphere the more the atmosphere resists. It gets less and less efficient the faster you go. That's just how it works.
The Concorde engine was efficient at cruise - and pretty awful everywhere else. At cruise it was about 10% more efficient than a GE90. But the Concorde had 4 of them, and a 777 has 2 GE90s.
This is also why you don't see 4 engined planes anymore, except for the A380 and most people were hell bent on retiring during the pandemic until they couldn't get more 787s/A350s and the 777X is delayed.
This is actually not even approximately true; like many aerodynamic phenomena, drag is super nonlinear in airspeed and not even monotonic, with a bunch of weird outcomes around mach 1. The first chapter of every aero textbook probably includes a variation of this famous chart [0], showing drag coefficient or sometimes drag force (estimated for some famous/classic airframe) as a function of mach number.
This is why prolonged supercruise can exist for airplanes without giant comical fuel tanks etc.
That said, your main point about CAD/physics definitely stands and is a good one; imho it would be unlikely to design and build e.g. a 10x better airplane than the Concorde today, even with all the fancy computers simulations etc.
[0] https://www.researchgate.net/figure/Drag-coefficient-as-a-fu...
Computers are great for marginal gains and saving labor costs, but they can't give a 10x improvement unless there's already a 9x improvement in materials and techniques. Small nit, but CAE is simulations, while CAD is just drafting.
Even if your drag coeficcient goes down from 0.3 to 0.25, doubling your velocity will still more than triple your fuel consumption over constant distance. Then the tradeoffs in airframe to make it not fall out of the sky in the transonic regime, then the loss of ISP from much lower bypass engines, then the smaller cabins, then carrying the extra fuel... you can carry far fewer passengers. Sure your drag went down substantially from mk 0.98, but noone is proposing doing a transonic trip because that would be insane.
You wind up using many times more energy per passenger, meaning you pretty much only have first class due to noone on a budget even considering paying 10x as much. Throw in the cost of SAF and you're looking at the price of a house per ticket in order to save 5 hours in the air from a total 15 hours travel/boarding/waiting time.
You are making two assumptions, one explicit and one implicit. That the cross-section area is constant (it’s not, it depends on the angle of attack, which is smaller at higher speeds) and that the air density is constant (it’s not, supersonic planes fly at much higher altitudes, where the air is thinner).
Of course it would be less efficient than a subsonic jet, but they never claimed the opposite - the idea was for it to be faster, but not (much) more expensive than today's business class on subsonic jets.
