A couple comments on supersonic aircraft:

There are no jet engines ever made that I'm aware of that can run at supersonic speeds. The fastest mass air flow rate I've ever personally heard of was some Soviet thing that didn't immediately self destruct around mach 0.98. The way supersonic aircraft generate thrust while supersonic is the inlet geometry does WEIRD things to turn supersonic cold low pressure air into low subsonic hot higher pressure air. Most jet engines are happiest with inlet air flowing around 200 to 400 mph. If you run a jet engine above some critical subsonic speed occasionally shock waves will impact the compressor and every time the compressor rotates the blades will hit those shockwaves, and they rotate pretty fast... So the MTBF for a SR71 compressor blade might be 100000 hours at a nice subsonic mach 3.25 but whack it with supersonic shock waves every rotation and at mach 3.5 the MTBF for a compressor blade might be one hour. The nozzles in front of the engine fully retract at mach 3.25 and I'd interpret that as the inlet system is not designed to run "much above mach 3.25". There's a story about a flight over Libya where the pilot pushed a SR71 to mach 3.5 and that engine was likely pulled and sent to scrap yard after that stunt.

The other side of the problem with inlet geometry is the inlet temp gets very hot so you have to go ramjet to get most of the thrust because trying to generate significant thrust will melt the turbine blades off. So hypothetically if you got 10K pounds of thrust by increasing the temp of the air by 1500 degrees and the inlet was dumping 800 degree air at mach 3.25 into the engine, the combined exhaust temp would be 800+1500=2300 degrees which is getting hot for turbine blades (all numbers made up). So the second engineering limit on why the SR71 never flew at mach 9 or whatever is based on public engine designs and the temperature of mach 9 air when slowed to subsonic, there are no metals you can make a turbine out of. In fact at mach 6 or whatever its kinda mysterious how they keep the ramjet from melting itself. Must look like a rocket engine with cooling passages for fuel everywhere. So based on the published engine design (and is the published data trustworthy?) there's no way to bypass enough air to not melt the blades off the turbine above mach 3.something because nobody has high enough melting point metals. As with the compressor situation I'm sure a 3.25 rated turbine blade that has a MTBF of 100000 hours might survive an hour or two at 3.5, but the engine is scrap after that flight.

The third supersonic comment is mach number is not constant with altitude so a pessimist person could say a mach 6 that flys high enough could very well indeed have a slower groundspeed than a sr-71 or even a 747. If the speed of sound at 120Kft is like 190 knots, then mach 6 at 120Kft would be a ground speed of merely 1140 knots. Famously the SR-71 could achieve ground speeds of 1800+ knots (supposedly...). So its simultaneously true that a SR72 might run twice the mach number but if it runs at a high enough altitude like my 120Kft example, its possibly only 2/3 the ground speed of a SR-71. The flight envelope of the SR-71 from the declassified manual is available; note it could not run supersonic below 20Kfeet or so. This also comes up in discussion of interceptor missiles; there's a huge difference in speed between a plane that can't go supersonic below 20Kft and a SA missile that can go mach 5 or whatever at ground level right off the rail. So its not as simple as "a mach 4 surface to air missile can never hit a mach 6 airplane" because mach 6 at 120Kft is much slower than mach 4 at ground level.