The SR-71 did end up being shortlived because SAMs that could catch it were developed only a few years after we started flying it along Soviet borders, but when it was new there wasn't anything that could catch it as I recall, but I could be wrong about that, and also maybe they were being cautious since the whole reason the U-2 was shot down was because we (falsely) believed they didn't have radar capabilities that could detect it when it first started flying.

Mig-31 at least definitely didn't appear until much later, and the Mig-25 could only catch it on paper, since while it had been observed going over Mach 3, we would later find out it wasn't capable of sustaining that speed, whereas the SR-71 was. It's mentioned in the video I think.