The changes to mana and piety are almost certainly there to make it so that healing in FFXIV more resembles other MMOs. In FFXIV healers have some limited heal resources (Essential Dignity has a cooldown, aetherflow has 3 stacks per minute, ect), but for the most part you can constantly heal. Like I tested it on scholar and you can toss out AOE heals non-stop 33 times and your expensive heal 22 times, which is... a bonkers amount of healing before running out of MP and having to slightly slow down healing, even ignoring oGCDs and the fairy.
Healing in most MMOs is an on the fly resource management game, and while it may not be to everyone's taste, the 'mainstream' healer demographic is a very important one FFXIV doesn't cater to at all, and that is a pretty big problem. "Do I regen this DPS who took incidental damage, do an efficient burst heal, toss out a mana chugging AOE heal?" is an interesting questions in those games because going too light might result in their death, going too hard means you run out of mana faster and may be left without healing when you need it. That... can't happen in FFXIV, so the only real skillful play is knowing when you need to heal at all, knowing when you can stand still as a white mage, and knowing how to weave energy drain if you are a scholar. Not... a lot going on, which is why healers going from other MMOs don't often heal in FFXIV. Ironically, because healers are so insane at healing to the point you can always toss your biggest GCD heals whenever you want, they actually are less interesting than when they have mana problems. So I can fully see why there is a double standard and why healers won't be able to meld piety to make their mana efficiency better: Because healers being very mana efficient actually makes healers LESS fun to play.
Course there are other considerations: other games have healer 'downtime' where they can catch their bearings and monitor the team, and while FFXIV has plenty of downtime too usually you spent that DPSing, so hopefully healer DPS abilities won't cost mana. Likewise, while potency adjustments were necessary, as well as nerfs to scholar, it is sub-par an enjoyable part of the scholar kit of oGCD offensive weaving is going away. Those are some weird changes and will leave a hole in the game if healing doesn't get a lot more active, and more active healing means more folks wiping in roulette content, and roulette content being a more serious barrier to progression is bad with how long it takes now to get to endgame.