At gear parity in leveling dungeons all the healers have to be able to use pure heals. if they really wanted to go into this whole shielding business they'd need an entirely separate role for it, the party size would increase to 5, and content would have to be readjusted just to handle the fact the party only has one real healer anymore in the pure healers.
In DnD a shield is really just a pool of temporary HP that drops after a time and is available on classes that are not even technically healers, and in truth I've never used any of them because the only point to having a shield is that you're predicting an attack is going to hit you for more than your maximum HP. How would we know that is going to happen? At what point would it be superior to just using damage reduction?
Temp HP does give another lever to mess with when balancing out a defense kit on a job, but its basically meant to be a worse damage reduction and should be used that way. The main use it has from healers in FFXIV from my observation, is to allow healing over time to recover the real HP while the fake HP is widdled away. All the points where we got to use shields with mits and full HP are are from over compensating attacks that shouldn't exist.
That also goes into the design decision where we are now supposed to use damage reduction and temp HP in response to an advertised attack, rather than rotating the damage reduction and temp HP to help keep the tank up through all forms of content. This whole change led to a massive amount of technical debt that is taking them years to fix, and even if they fix it, will likely make all new content feel stale very quickly. If it isn't possible to randomize when big hits happen and all big attacks must be advertised within the 2 minute window, then all fights are going to start looking the same. They can change the aesthetics, but they have no flex for timing. And with greatly simplified jobs that is going to make the game get boring rather quickly.
Difficulty of content is completely determined by the gear score someone is wearing, how much they obfuscate mechanics, and how many people have to participate in the mechanic, with all mechanics and attacks timed to a 2 minute window. That is the game we are playing, and likely the game we are going to be stuck with.
Also, I feel that all the changes made are to make healing less random and stressful because traditionally, healing even as a firehose healer in other MMOs is kind of a stressful babysitting job.