I mean I'm not saying there aren't design issues here.

But going down the spec route will inevitably result in "bring the minimum amount of healing necessary to clear, and maximum damage." And even if SE committed to reworking every dungeon and nerfing tanks on a scale that the game hasn't seen in its entire history, it will still be a pipe dream to expect every instance to require such an amount of healing that someone would go all in on a healing spec.

Heck, even in ultimates, people would be going "Well you don't really need this ability or that ability, you can cut it for more damage." We pretty much already get the FF14 version of that - figuring out where to cut healing for that extra damage GCD.