Interceptors in general were made obsolete by the time the A-12 and F-108 programs were being considered and ultimately canceled.
The first reason was ICBMs becoming the dominant form of nuclear deterrent. The A-12 was never going to be built into an interceptor that could shoot down an ICBM.
The second was the Nike Hercules program. They took the Nike surface to air missiles, enlarged them, and put a nuke on top of them. This did a reasonable job of solving the ICBM problem, and also solved any problem that would have been solved with interceptors. There were 200-300 Nike Hercules sites in the continental US, plus a non-nuclear export version that had a 1,000lb conventional payload which was deployed in many other NATO/US allied countries.
The end result is that even though there's still a need and a market for air superiority and multirole fighters, the high speed interceptor is not a role that exists in a modern military anymore. Much like the battleship for instance.
(edit: note that the fastest strategic bomber built by the Soviet Union was the mach 2.05 Tu-160, which is slow enough to be intercepted by the mach 2.5 F-15 or the mach 2.4 F-14.)
Reagan "Star Wars" 2.0 energy weapons will be the ultimate anti-ICBM, anti-air, anti-tank, and anti-personnel defenses. I can't see crewed bombers or fighters surviving much longer when lasers and UAVs make them moot.
Can we not now envision threats moving faster than a Concorde?