The SR-71 was replaced by several drones. One of the problems
with the SR-71, was the Intel was basically redundant to what was
collected by satellites. It had no tactical intel value due to its speed.

Operating out of the former Soviet republics, we have been flying
drones almost every day, producing real-time data, and electronic
order of battle tables, that used to take weeks with the RC-135.

Speed isn't everything, and it isn't the only thing. Most battle
managers would love to just park a vehicle over a country and
have it update in real-time, rather than the one pass a day, the
old SR-71 provided. There's drones today that have unrefueled
orbit times of 24 hours, and they have kilowatt power sources that
are able to produce significant data collection.

The unsophisticated days of Vietnam, with its morning recce,
followed by an air strike, is over. Even the most basic forces have
much more sophisticated defenses, and it takes hours of
observation to produce an ATO.