Enmity build up on untargetable bosses

[Eisenhower]

Why make a "Snip" quote if you're going to refer to each section of the post in detail again later in the same reply? It seems... superfluous. And there is no need to be condescending. But I digress, it appears the key point of our disagreement on this topic is that you consider the party to be still fully engaged to the boss when it is displaced, whilst I do not.

[...][1] You may consider it a conjecture, but past content proved otherwise.
[...][2] Healers have to keep the party in good shape throughout the phase, so why would they be the only ones who would build enmity by doing their job? Explain this: Why is this even a thing?
[...][3] Otherwise there's no management involved for anyone if they can't do anything about it. You can only stack a certain amount of enmity and with it comes a drop in performance on tanks or drop in performance for healers if enmity becomes an issue [...]
[...][4]I don't think anyone is a big fan of jobs that's practically clones of each other.

(1) Existing content do show that mechanics are recycled. It also shows that certain mechanics have not resurfaced yet, among which the "Sephiroth Dive" which is one of the key troubles you highlighted. Although we are not at fault in any way for speculating, all current data point towards that mechanic not resurfacing, and with that the notion of mechanics tailored to "punish" healers diminish greatly (be they current or future).

(2) I mentioned this in point 2 previously. Observe the image below. When the boss is displaced, interactions labelled ① cease along with the capacity to manipulate enmity directly via abilities such as Provoke. However, the interactions labelled ② function unhindered and from that I argue that the entirety of that interaction should remain - i.e. a healer still interacts with a displaced enemy, and what responsibilities may come from it. (Technically, ninjas also play a small and indirect part should they want to.)

When a boss displacement phase introduce additional enemies, the image applies separately for each present enemy, allowing full interaction between the Tank/DPS and the paired enemy. That some enemies can cause enmity build-up in fellow enemies according to certain rules may very well fall under encounter specific mechanics.

(3) I'd like to point out that there is a way for players to manage enmity even in displacement phases. The tools are few (or actually singular now that I think about it) and have a limited impact, but to claim that no management exist is simply false.

Moving on, am I correct in inferring that from your statement you consider tank performance to be more strongly dependent on their damage dealt, rather than their ability to keep enmity and mitigate damage? Assuming that is the claim, isn't it more reasonable that a tanks performance should be gauged on their ability to balance damage dealt with enmity generated? (After all, the tanking/raiding meta may change) If damage dealt becomes their main metric of performance, aren't they effectively a DPS class?

(4) It is plain to see that some jobs are not as well put together as others, both in terms of how well each individual piece of the toolkit performs as well synergy between these pieces and in comparison to other jobs' equivalent tools. I too dislike the level job homogenization present in XIV and though ASTs would most likely enjoy a Shroud clone rather than a Quelling clone when it comes to enmity management I think the best would be a more unique ability designed with consideration to what and how the job performs.

Coming from that, I get the feeling that the actual issue at hand is that ASTs (among others) has poor internal synergy which symptoms manifest in certain encounters? Wouldn't it be better to correct the function of astrologians rather than change encounter mechanics and enmity feedback? It would retain a layer of gameplay considerations and provide the tools to solve the challenges presented by that layer. For example (and a poorly constructed ones, I suspect), working from the assumption that "Sephiroth Dives" will not reappear, but displacement enmity is a thing:

LA's duration is much longer and refresh potency is lowered to achieve the same overall mana return. OR
LA now instead reverses enmity generation when active. OR
LA instead halves current enmity but returning it over a period of time.

The point is, there are ways to address these topics without simplifying the game.