Some theory: They would not have retired the SR-71 without something better and something non-satellite, so something non-predictable.
For stealth reasons some smaller drone might be better. Not sure if speed is the advantage it once was especially with all those tankers and inflight refueling involved back then. The question is why would anyone today still want it to be manned? Signal delay? Why not a smaller drone? Like folded up in a box dropped from some innocent C-17 inflight or something you can launch from a submarine?