[Dennip]

IIRC The SR71 stopped being useful as a spy plane because missile defence improved to the point they could be shot down, and spy satellites took over the duties of reconnaissance.

These old airframes do get incremental upgrades to software/avionics/engines etc, and there is actually a SR-72 in development as a UAV.

[I_Am_Nous]

From what I've read, the SR-71 missile defense system was just to throttle up. Back when it was the fastest bird in the sky, this worked well, but we have supersonic missiles that can probably match its speed, and hypersonic missiles are even starting to be used in Ukraine.

[ceejayoz]

The MiG-31 largely stopped that approach from being viable. https://aviation.stackexchange.com/questions/16377/could-a-s...