If AI starts to make stealth less effective, then it is a return to speed. Perhaps one could use the Blackbird’s proven airframe to test new propulsion systems in the speed range between Mach 3 and hypersonics - which eliminates the problems of the old engines and special fuel.
Ummm...you know there is stuff out there that can intercept missiles now? And some of those missiles go a lot faster than any plane ever has, so a really fast plane isn't the invulnerable piece of kit it was 50-60 years ago.