How could you do this to AST?

[MintnHoney]

You mean the Pigeonholing that has been there since AST's release? Nocturnal Sect has always been a trap due to being fundamentally weaker than SCH, and AST's abilities since Stormblood have been pushing it into a pure healer role. The devs are unable to effectively balance AST without causing major issues in the healer balance. Make Nocturnal too strong, you throw SCH out of the meta. Make Diurnal too strong, and WHM is forced out. Make either too weak, and AST can be tossed out.

That has been the flaw since release, and anybody who has been here since HW can recall those days where they were basically "rebalanced" almost every patch. It caused massive chaos. Even in SB you saw AST's balancing caused chaos. Here in ShB it too caused chaos. Let's just be honest with ourselves here, AST has been in a dire need for a rework since release, simply because the Sects made it impossible for any semblance of healer balance to exist.

Technically, it's been there since before Heavensward's release, and it worked then, because there were only 2 healers (so making that distinction between them was an easy way to make the gameplay different and, therefore, more fun and unique).

You're asking me to be honest about AST needing a rework, but I'm not disagreeing about that. I'm actually of a mindset that AST should have an even more drastic update, even so much as a role change, so that they can, potentially, be able to stop totally fucking around with the job every single expansion.
However, I don't believe that continuing the separation of the healer role into 2 specifically types of healers is going to solve the problem of "balance," wherein there will be pitchforks and tears the minute a job's competition becomes the "meta healer," in lieu of trying to allow all healers to be equally adept at problem solving the various healer conundrums that come their way.
It also does not necessarily solve the biggest problem healers have at the moment, which is what to do in their downtime. Yes, they still have time to make changes and we don't know everything yet, but, for me, doubling down on the concepts of "pure healing" and "shield healing" spells trouble on the horizon for any prospective idea that they understand why healers of various skill levels would be unhappy with the role at the moment.

But that's just my personal musings on it.