Yup, I'd say SR-71 was about as immune to aerial defence systems as U-2 was. Until it wasn't.
How many times SR-71 flew over actual warzone? How many SAMs were fired on it?
Besides, MiG-25 program gave Soviet Union fast, efficient and reliable long range interceptor and a very capable tactical reconnaissance aircraft. Remember, in the first Gulf War it was a MiG-25 that scored the first aerial kill, shooting down a F-18.
The SR-71 was decommissioned because satellites could do the mission cheaper.
Shooting down a F-18 is childs play compared to intercepting the SR-71. The SR-71 was easily twice as hard to shoot down or more, they are not even in the same league. The U-2 and Sr-71 usually flew within 10,000 feet of each other over hostile territory. The Sr-71 was traveling three time faster and could easily change its speed by hundreds of mph in a short time.
Perhaps maybe....
If the Mig was in position...
If the Mig engines could be written off...
If the missile could lock on and track at max range...
If the missile could maintain a solution up to Mach 8 closure rates....
The main air to air missiles at the time had only a .3 to 1.0 mach advantage, making a tail or side profile shot nearly impossible. In all the SR-71 was fired at hundreds (approaching 1,000) of times without a single successful attempt, save a few holes on a post inspection walk around.
Strip