The issue with that ideal is it is absolutely not what the fight Design in FFXIV is built for.
Tanks at least still have tankbusters and tank swaps in savage to deal with, on top of getting dps rotations of varying complexity and apm.
DPS (well outside Summoner) get Interesting rotations with more or less room to optimize. And dps checks to Beat.
But healers are designed for dealing with fights we never See.
For one there's no mechanic I can recall in content I play (which tbf does not include ultimates or criterion) that needed non-tank spot healing.
There's no random dps chunking mechs you need to spot heal asap bc that dps getting hit by it again would kill them. It's always predictable and usualy party wide dmg.
Even on Tanks the healing needed is very predictable. Unless it's a tankbuster, or you cheese some mechs with the 'drop to 1' invulns, Tanks can wait for healing until you'd need to hit an AoE heal anyways for a raid wide.
There's also very few mechanics that actually need a lot of Healing through put to deal with. HH is the only one that jumps to mind this Tier, with styx as a runner up at least when the parties gear is meh.
But looking at the toolkits healers have rn you'd think they'd need to deal with raidwides needing heavy mitigation every 30s or so. Or ones needing a single healers mitigation every 15s. Tankbusters on both tanks about every minute or so. Etc.
And even then, there's few mechanics in game that would allow Healers to show their stuff beyond a binary 'fail to heal and wipe' and 'heal enough to pass'.
These days there's basicly no tankbusters in ffxiv where the party has a choice of when to do them. Imagine if the vulns given by tankbusters were less 'next hit instakills guranteed' and instead just ramped up all dmg taken by the Tank. So that skilled healers could drag that tankswap out longer than average healers. Potentially even so far that you could skip the Tank swap with two good enough healers and thus freeing up the second tank slot for a dps to speed up Runs. Or maybe even give the Off-tank a dmg buff equal to the vuln stacks the Tank has to just speed up the boss even with a standard comp the longer healers can keep the mt alive.