Player Drop Off and Accessibility/Diffculty

[Absurdity]

As for tanks missed mitigation usually does wipe out the party so i don't agree with your arguments.

If we're talking about the usual party mitigation from tanks then yes, the party just dies if you don't use it when you're supposed to, there is no decision to be made.

Stroodle is most likely talking about the short cooldown mitigation that every tank has.
Since tank damage is a joke in most fights you only need it for a tank buster, which leaves you with a lot of opportunities to throw a Heart of Corundum or Nascent Flash on someone that's too low on HP to survive the next raidwide.

Unfortunately that situation is rather rare.
If a DPS does a mechanic wrong in savage they're usually just dead.
And if it's the expected damage they should take from a mechanic then it already gets covered by the healer's planned AoE, since modern mechanics pretty much always hit the entire party and everyone needs healing anyway.

It used to happen a bit more often that someone was lower HP than expected because the heal didn't catch them, before they buffed healer AoE range into outer space.
But notice a pattern here, everything that could potentially have lead to player skill expression got patched out of the game at some point because it was deemed "inconvenient".