The SR-71 is because they need to test for speed.

That serves a huge number of things both offense and defense: engagement, radar, missiles, trying to boost the speed capabilities of new planes & weapons. And it applies both to manned and unmanned planes.

Russia is less of a concern now, however China is coming on very fast in terms of leveling up their military capabilities. I'd be very surprised if the US military isn't looking to push its capabilities around increasing speed generally, just as it does any other aspect. There hasn't been a significant leveling up in US Air Force speed capabilities in many decades (since the F22 supercruise three decades ago?).