The S400 and the Patriot etc all suffer from the same problem: They only deny the enemy high elevation flight.

Russian doctrine has been based around big, fast things going high. NATO doctrine has been about smaller, slower things going very low. Going low leaves you very vulnerable if you get too close to a defender, but there's no way there are defenders everywhere.

An extreme example of the problem was the Moskova--big, fast missiles that couldn't see something coming in just above wavetop height. There were only two launchers that had any possibility of engaging and only time for one launch cycle--and that probably only if they already had a bird on the rails. (Exposed to the elements, rather than safe in the magazine.)

The SR-71 wasn't trying to catch the MIG-25, it was trying to get away--and it worked. The U-2 proved vulnerable to filling the sky with cheap stuff--the missiles were ballistic by the time they got up there but when the sky was full of them the U-2 had no path to safety.

The SR-71 couldn't be defeated by the level of missile spam that Russia was capable of, the MIG-25 couldn't get close enough to catch it and they didn't have a missile that could actually work up there. (You need more control surface up there, but down lower more control surface costs you performance.)

(And the MIG-25 was a maintenance nightmare.)