There were armed variants like the YF-12. So far as I understand, the main reason the game changed is because missiles improved. Not much point focusing on things like speed when some missiles are reaching mach 16, contrasted against the SR-71's mach 3.3. In that context you simply have to pray your counter-measures work, and if that fails, then hope your ejection seat works.
Bullets are slower than the SR-71 - it would shoot itself down if it had a gun.
Newton disproved that more than a century ago.
Did you mean Einstein? But it was only a “thought experiment” …