To be fair, it was better before. I could see a point getting more MP,especially when Lucid wasn't a thing and we relied on Ranged/CD for MP regen. Now,it barely affects anything. No real point building some piety.
I don't know, if they made Piety to be Healer's Direct Hit equivalent, that would be nice.
I don't know. I think it was a mix of multiple things that happened at once.
There's this obsession to remove player decision making in some sense. SCH using macros to force Embrace? Can't have that. Remove all pet actions. DRK player using /ac <me> to put Salted Earth to the ground rather than do it manually? No, can't have that. Remove it posthaste. Astrologian can draw cards by left clicking them? NO, USE UNDRAW! It's just weird decisions that nobody asked for, and just rob current players of bits and chunks of their gameplay ( well, in SCH's case, almost half of it ).
There's also the slow removal of DoTs since Stormblood. AST had the real first taste of current ShB/EW gameplay - a nuke, a dot, and whatever is going on with the cards. Even at the time, people were complaining about this design because while AST was good, it was pretty boring. Sure, 2 DoTs weren't much to begin with, but it was something to manage, at the very least. What was SE's decision to do next expac? Apply that gameplay on all healers. Aero III was fun. The animation was great, and while we didn't have Bane, it was satisfying to apply on packs in dungeons. Nah, remove that.
Bane? Remove that as well. Pet management ? To the bin. Buffs variety? Cremated. Buff management with Time Dilation, SCH's AoE Esuna, Rouse, Protect, Stoneskin, etc etc? Done. Energy Drain for a month? Gone. SCH was a skill graveyard when ShB started.
And on top of that, there were some kind of promise that we would get harder heal checks coming over, and that never happened either -- granted, Tower of Zot and Holminster were both a bit harder than expected at release, that promptly disappeared.
tl;dr removal of player choice, removal of dots across the whole game, hitting healers the most because they relied on it the most, trying to apply SB AST gameplay to all jobs despite not being fit to, and no change of healing in either dungeons and harder content.