The S-400 is taken out by launching enough armed drones against it that it uses up it’s supply of missiles defending itself against them... it has to attack the drones or die.
To be effective you would need enough S-400 missiles that an enemy could not launch enough drones to deplete them.
Well put. And it isn’t worth a damn against a ground attack. Also, like the USSR before it/him, Russia/Putin tries field tests in the Middle East because that’s where their systems will face the greatest challenges.