One problem which I've heard about on supersonic airplanes, particularly ones that spend long amounts of time at supersonic speeds, like supersonic bombers (XB-70, B-1A, Tu-160), high-speed recon aircraft (SR-71), and supersonic-transport concepts (L-2000, B-2707, HSCT, AST).
Now, I know airframe shape plays a significant role in the kinetic heating of the plane. A highly-streamlined clean design doesn't reach as high a temperature for the same mach number as one that's not as cleanly designed. But I'm wondering if it's more than that why fighter planes can do Mach 2.5 or so with aluminum skin and an SST to achieve Mach 2.5 would need at least some titanium in it's design. Especially considering that fighter planes actually are not as cleanly designed as a larger supersonic aircraft to the best of my knowledge, relying more on extreme thrust to weight ratio than extroardinary aerodynamic cleanliness except some old interceptor designs not that they are high-drag designs.
My only guesses are that
-Fighter planes are a hell of a lot sturdier, with thicker skin, and a thicker internal support structure -- they're designed to pull 9 G's or more, in the olden days it was 7.33 G's: Way more than an airliner which requires at least 3.75 to 5.0. The guess is that since fighters are so much sturdier, that even as they weaken a bit, there's still so much strength there that it still will hold up until a higher speed.
-Large supersonic-aircraft fly much longer at supersonic-speeds than fighter-sized and as a result fully heat-soak when fighters don't. But the thing that has me wondering is if fighters are smaller and less massive than the larger planes, wouldn't they heat-soak faster?