Radar evading stealth is partly a myth and technology exists to beat it to a large degree now.
However physics is physics and a very high very fast target takes a huge amount of reaching. Even simple manoeuvres by the target are difficult to counter.
SR 72 looks to be an unpiloted, cheaper and less complicated son of the Aurora black project, which had a relatively short service life. Probably due to technical or budgetary problems.
Control technology has come as far as the X 45, where the vehicle is autonomous. It is not "flown" from the ground, just told what its mission is. This is ideal for high speed, high altitude reconnaissance.