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If I remember correctly, Russians had foreseen the saturation attacks. They have come up with a different solution i.e. IADS(around 2008 or so). I do agree that even IADS will not guarantee the invincibility on the battlefield. But the cost to take down one S-400 would increase exponentially.
Now add the quick deployment feature of S 400 and other Russian SAM systems. I hope you've read about Kosovo war and how SAM systems slowed down US operations. Imagine equally trained enemy operating mix of S 300/400 with BuK and Pantsir systems. Enemy military planners would face serious pressure to plan their missions, not to mention risk and effect of morale when fighters.
The first wave of attack against modern SAM sites most likely will not be kinetic, it will be EW or virus to seize control of the radars, trick or jam it.
Followed by saturation attack by Tomahawk cruise missiles to both disable known radar sites and force AD operators to turn on secondary SAMs. Stealthy JASSM can also be used to attack high value radar sites. Then next wave will be a mix of B-2 (or B-21 in the future), F-35, MALDs (as decoys or EW), suicide drones, Growlers, F-16CJ, Tornados and B-1s acting as bomb and cruise missile trucks from stand-off distance. Every time a radar turns on it will be supressed.
The primary SEAD weapon of choice after saturation cruise missile attack will be AARGM. Within seconds from the time ground radars are turned on SEAD planes will launch AARGM. SEAD forces can also resort to shorter range A2G missiles, SDBs, glide bombs to go after point defense radars that dares to turn on.
To conserve their AD assets, red forces will restrict turning on their radars much like what the Serbs did in 1999. This alone is already a tactical advantage for the attacking force.
It will depend on who is using SAM systems. If it is the Russians they can definitely mount a robust defense (not insurmountable but attacking them will cost a lot of coalition assets). But any lesser operator will easily be clobbered into ceding the airspace and resorting to hit and run tactics or Hail Mary SAM launchings.