I don't think it's a good idea to consider 'SE can't code without making a spaghetti mess' as a factor in whether an idea has merit or not. If we did, nothing would ever get added again, look at how jank most additions to the game have been. Glamour as a system was a hack job back in 2.3 and we're still paying the price for its implementation, for example, and since then rather than fixing it to work better, they've bandaid fixed the storage issue with Glamour Dressers. Nor do I think 'what if one job is better at solving the mechanic than another' is a factor, because A: To avoid 'one job is better than another' you'd have to have every job be entirely identical in their designs, and B: SE has made situations where one job is clearly better than another at solving a mechanic quite regularly, so long as every job can clear I don't think it matters so much if one job has a slight advantage at a mechanic. Look at AST in P3S vs WHM (which actually brought AST play rates up to WHM levels for that one fight), look at a great many fights that have 5 tankbusters, where WAR (and only WAR) can Holmgang 3 of them (this happened again with M3S, so it's not like that was just a Stormblood thing).
I hear a lot of people saying that 'the solution to make healers more engaging is to make us heal more'. The idea I posted would make people either heal more, or use Esuna more, both of which are 'what healers should be doing' according to some players. It being a separate system, I'd argue, would allow for more granularity on adjusting how much damage it does/how hard the fight is, not less. If SE were to just do a change like 'nerf healing power by 50%, nerf incoming damage by 25%' (such that we're effectively weaker at healing vs now), I think that runs at least the same risk of 'fight is too hard' (if not more), but also there's less dials to adjust to change the fight difficulty to a more satisfying level than current gameplay (but still clearable). Consider, as we get more gear currently, the healing challenge dissipates, because our heals get stronger AND the damage we take goes down (due to Defence/MagicDefence stats on gear). But an external mechanic like the suggested one, doesn't necessarily have to interact with the defensive stats. The mechanic that applies 50k of Aetherblight in Week 1 progression, could still apply 50k in full BIS, our healing power would go up, but the value to heal would not go down (unlike currently), making the content more resistant to 'being outgeared' in that regard
At this point, I don't even think complete removal of Tank/DPS party-mit/selfheal tools would solve the issue. Take WAR. Back in HW, they had Equilibrium (which they didn't want to use because in Deliverance it restored TP) and Inner Beast (which they didn't want to use because that's a Fell Cleave you're missing out on). If WAR were taken back to that point in terms of their party-mit/self healing capabilities, I don't think that really solves the Healer gameplay issues. Instead of using 5 of my SGE tools to get through a dungeon pull, maybe I use 7, or 8. But I have like 9 total tools, so my gameplay is still 'OGCD bullshit go' and then spam Dyskrasia. We can't just blame it all on BloodWhetting and the like, our own Healing kits are part of the problem too. I mean, look at DRK, back then in HW they had Dark Arts Abyssal Drain, which was functionally similar to what we now know as Bloodwhetting. But it wasn't really an issue back then, and it IS an issue now. What changed? How much extra stuff we have access to with an additional 40 levels of kit would be a factor I expect
That's a lot of healing and mitigation across that party, yes. Now imagine we remove all of the mits, and raidwide healing actions from that team. So, WAR loses SIO and Nascent Flash (they can only heal themselves with BW/Equil), GNB can only self-target Aurora and loses Heart of Light. Monk loses the Earth's Reply heal (it can keep Mantra since it's been there since ARR), PCT loses Star Prism's heal and Tempera Grassa (it can keep the self-shield), DNC loses Curing Waltz, Improvization and Shield Samba, and RDM loses Magick Barrier's 10% mit (it can keep the healing+ since it's basically another Mantra). Okay, with all of that stuff removed, how do I as a healer, tackle Worqor Lar Dor EX? TBH, basically the same, but I press a Medica3/Aspected Helios every now and then to keep up with the DOT.
So, after all of that, the net result of the changes listed is that our gameplay now includes an occasional Medica3 that wasn't there before. I don't think that'd really solve much. Plus, old content has been designed with those mits on non-Healers in mind, so removing them means having to go back and check old content to make sure it can be cleared with the changed job kits, and the added pressure on the healers to do all of the mitting/shielding, and that sounds to me like it might cause 'risk of mistake making some fight too hard and being nerfed later'