Enmity build up on untargetable bosses

[Lyrica_Ashtine]

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

I do hope you realise you're not even in line with yourself here. Not addressing this would involve:
Tanks spamming the enmity combo over and over
Healers still need to heal a certain amount throughout the fight. The only way to avoid additional enmity is to just sit there like a sack of potatoes and not DPS.
The moment a job gets issue with this would require retooling

Spamming 123 over and over for the enmity combo seems even more simplified. Or healers not DPSing to avoid enmity makes it a simple press-1-to-heal job. If anything that would make it oversimplified, it would be this kind of practice. Or would we have to resort to what you suggest; Retooling? whenever a tank or healer - or just healers specifically - would have problems concerning enmity, Square-Enix would simply need to retool their kit? I believe this is far worse practice than what you're trying to sell:

From that, we either end up having to change AST and in a sense make the displacement phase change superfluous or play a game where such mechanics have no place because of the inherent weakness of AST. The latter is a bad place to be in my eyes.

Let's say Scholars are the ones next who will have enmity issues, simply retool their Aetherflow as you propose? Or if a certain tank has trouble keeping enmity on any healer due to the phase, simply retool the tank? Constantly retooling everything won't solve anything in the long run. Square-Enix will simply reiterate this whole endeavor over and over while removing this whole "displacement-but-you-still-build-enmity" gimmick is solved once and for all for any role and any encounter. Is it that hard to understand how nearsighted your idea is to simply retool jobs over and over? Astrologians are the one in the spotlight concerning this gimmick, with Paladins somewhere in the background. So what if a new tank/healer is introduced in the next expansion - With every current healer already retooled - and run into this trouble again? Retool them all over again till all the healers are practically clones of each other on the enmity board?