It's a good idea but it comes into trouble when say, if groups wanna go WHM/AST in endgame especially the ultimates. How will the mitigation be handled? A good amount of damage in those fights comes from sources that can't be debuffed directly. And there are some moves that can't be survived without shields(Gigaflare, Nael Megaflare and most importantly, Morn Afah) Not to mention, with the exception of Reprisal due to two tanks, the cds of the others are spaced out so they can't be up for every aoe attack. (Nael and Twintania in UCOB during the Bahamut phase and the primals in UWU during Woke Ultima)

I'm not saying the idea is bad but you need to consider elements of the encounter design and see what you can do to make it work.

There's also the fact about the lb generation you get from abusing shields so that aspect would need to change but of course we don't know everything yet til May so we need to wait and see.