"It is, in fact, impossible to reach the speeds of which the MIG-25 is capable using titanium: yet the Soviet designers had managed to build this, the fastest combat aircraft in the world, from ordinary steel. " Right... MiG-25, Mach-2.8. All steel. SR-71, Mach-3.5. Uses titanium for 85% of its structure.
Combat aircraft. SR-71 is a spy plane.
The YF-12 was a twin-seater derivative of the A-12 that was developed as a fighter aircraft, complete with a look-down/shoot-down radar and missile bays.
The F-12 never made it to production, but a small number high-speed of weapons tests were conducted with the 3 YF-12 airframes that were built.
With that said, the MiG-25 should certainly not be belittled as a marvel of aviation. It was a blisteringly fast interceptor and reconnaissance aircraft, with an impressive radar system for its time. It filled a special role in the Russian Air Force until last year[2].
[1] http://en.wikipedia.org/wiki/Lockheed_YF-12 [2] https://medium.com/war-is-boring/21d12739b5db