I have personally followed USSR radar station tracking of SR-71 USSR flyovers on multiple occasions in the early 1980s. At Mach 3 an SR-71 setting out from Japan didn't really have the capability to just skirt the outer layer of Soviet Air Defense without making an improbably wide arc; although I don't recall seeing any missions that took the Blackbird in as far as Moscow from the East where I was stationed. I never saw the Soviets solve the issue of successfully shooting down something going that fast, though I agree that anticipation of the development of the technology to do so is why the SR-71 was retired. I was out of the service by then, so I can't say from personal knowledge.
I'm not anti-Christian. I'm anti-stupid.
