The *only* thing you've said that is even remotely true is that mitigation isn't useful against unmitigatable sources of damage, such as on Kephess the Undying where *so* much of his damage is F/T. Of course, you're overstating the important of it since that's the *only* fight where you actually have to worry about it
"You only have to worry about it in the hardest fight available in this game atm".
I used to agree with you and KeyBoardNinja Kitru, but since EC NiM, I've changed my mind. You may end up slightly better off in most fights in the current end game with the mitigation stacking, but what you should optimize for is the hardest of them - the rest you should be able to handle anyway.
For the same reason that you don't consider HM FPs, I don't consider EV/KP (any difficulty) or EC/TFB SM/HM.
Mitigation is still important ofcourse, but that fight made me switch to 27B mods.