Mmm, I think the stack mechanic would still be worth keeping around. One of their stated design goals was to make classes have a simpler skill floor, but a higher skill ceiling. A good WHM with the stack mechanic might be familiar with how much damage is coming and know that, say, 2 Medicas + a 2 stack PI is enough to get everyone back together with minimal overhealing. This of course assuming we'd have a fight with the right amount of damage for that. Also, it's just more mechanically interesting than an oGCD AoE heal with a fixed potency. You could make everything in the game be just a button with a fixed potency but how interesting would that be?