[Aktungbby]

Of some interest perhps:

Even if claiming that the MiG-31 was one of the causes of the SR-71 retirement is a bit far fetched, it is safe to say that towards the end of the career of the legendary spyplane, Russians proved to have developed tactics that could put the Blackbird at risk.
The Mig-31 is still in service, but the SR-71 successor, dubbed SR-72 and capable to reach Mach 6, should be quite safe at hypersonic speed.

https://theaviationist.com/2013/12/11/sr-71-vs-mig-31/ http://www.dailymail.co.uk/news/article-2594839/Could-SR-72-Aviation-expert-claims-mysterious-object-photographed-flying-Texas-spy-plane-designed-cross-country-hour.html The pilotless SR-72, an aircraft being designed to fly at six times the speed of sound, will be able to cross an entire continent in less than an hour, according to Lockheed Martin.(who really needs people!)