The high altitude spy planes (U2, SR-71) were mostly to prevent being shot down; surviving to return the film to base.
Drones don't need to bring film back, and it's not such a big deal if a few of them are lost.
- There is no real need for SR-72 to be manned on all flights;
- Hypersonic plane would be very good for small (2m wingspan) drone delivery behind the enemy lines fast (in half an hour). This was already a concept with M-21/D-21, but now the drone part can be much smaller and one plane could carry more than 4 of those.
Satellites are quite limited when you need to make fast decisions. Hypersonic plane will move about 7 times slower than an optical recce satellite, yet it can maneuver very fast if you need to relocate the imaging area and can get at least 10 times closer to the target. Instruments can be selected/improved even after the first flight of the plane -- not true for the satellites.
Though I suppose that you could try to make your drones so cheap and in such volume that shooting them down becomes logistically and economically infeasible, but that's another strategy entirely.
The whole idea of the SR-71 was that it went so fast that SAMs could just barely not catch up to it before they ran out of fuel. Any other plane couldn't outrun them, but the Blackbird? Well, Mach 3.4 will give you some breathing room.
Missiles and radar tend to operate in a "bubble". Even with the Russian S200 system at 190 miles range, the time you spend in that "bubble" when you're at 70k ft is not often long enough to do anything. That also assumes the aircraft directly overflies the battery. You can play games like pointing the camera sideways and observing from an angle if you know there are going to be angry people with guns nearby.
I imagine thought a little snake left/right might compromise getting the images that was the point of the mission.