Isn’t the temperature on the body surface the big factor here? The sr-71 would heat up tremendously and it barely had any air pressure to contend with.
It's difficult but not a show stopper. Materials have come a long way. Also, a pre-cooled engine will be using cryogenic fuels so active cooling is an option, even desirable (some of these engines harvest energy from the precooler using turbines on the hot side). SpaceX thinks they can handle the thermal loads of Starship re-entry with ceramic tiles and stainless steel.
