Basically, satellites took over They are cheaper to operate and less vulnerable to defensive countermeasures and other failure conditions. From a Los Angeles Times article from '89 [0]:
> "The Air Force decision to retire the Blackbirds in 1990 is based on several factors. In congressional testimony, Air Force Chief of Staff Gen. Larry D. Welch identified the increased survivability of reconnaissance satellites, SR-71 vulnerability to the Soviet SAM-5 surface-to-air missile and the cost of maintaining the SR-71 fleet."
They took over, but there’s still an argument for spy planes; satellites arrive on their own schedule in predictable orbits. This makes it very hard to get timely photographs, and it makes them easier to defend against. A spy plane could get in and out of a trouble spot within an hour, all before the enemy can hide the nukes or whatever.
A modern spy plane however would have to be faster than the SR-71, which I don’t believe would be safe against modern SAMs.
Based on the totally amateur knowledge level of someone that once spent a few hours researching stealth satellites (MISTY, etc), there's obviously a demand within the NRO for satellites that can be launched into a known orbit, with published two line elements, and then go stealthy and change their orbit into something which cannot be predicted by enemy nation-state ground forces.
I don't believe anything worthwhile about current capabilities has been declassified, it's all conjecture by people looking at the X-37 and similar systems.
> "The Air Force decision to retire the Blackbirds in 1990 is based on several factors. In congressional testimony, Air Force Chief of Staff Gen. Larry D. Welch identified the increased survivability of reconnaissance satellites, SR-71 vulnerability to the Soviet SAM-5 surface-to-air missile and the cost of maintaining the SR-71 fleet."
[0] https://www.latimes.com/archives/la-xpm-1989-04-09-op-1582-s...