FFXIV’s combat never had elemental resistances after A Realm Reborn came to be. When 1.0 died and 2.0 was launched in its place, there were remnants of the old elemental resistance mechanics that were present in 1.0 (materia, elemental resistance stats on your character, elemental resistances on ARR gear pieces), but they were so watered down that attempting to stack any sort of elemental resistance in the player’s favor was negligible—and all aspects of the resistances have been removed. Combat itself never relied on the old elemental resistances seen in single-player Final Fantasy games—possibly because there were only two classes/jobs at the time that dealt elemental damage (CNJ/WHM and THM/BLM), so it would likely be a system that wouldn’t work for any other class/job when faced with magical opponents.
Jobs used to have STR down and INT down mechanics: PLD’s Rage of Halone used to put STR down on bosses, MNK’s Dragon Kick was INT down, and DRK’s original Delerium was also INT down. There also used to be resistance downs such as slashing, blunt, and piercing—but a lot of these caused balance concerns, particularly piercing resistance down. As a former BRD main, I don’t miss the inherent reliance my job used to have on having a DRG present, even if I did like the synergy of SB BRD with DRG’s Battle Litany (and SCH’s Chain Strategem). That a lot of these were removed were due to the balancing issues they caused. Which were partially on the developers, since Yoshida outright admitted that they never balanced BRD or MCH around piercing resistance down, and scratched their heads each expansion when the two were doing far more damage than they anticipated.
None of that was why the old card system didn’t work, though. Cards didn’t affect elemental resistance or offer debuffs or anything like that—they purely buffed the party (either offensively or defensively). I don’t know how familiar you are with the old system, but they were as follows (single-target with no Royal Road enhancements on them):
Balance: +10% damage increased (was momentarily +20% damage from Patch 3.4 when the devs were desperately trying to make players play AST)
Spear: +10% critical hit rate (used to be reduced cooldown duration for buffs used while under the effect, but was changed because Spear was Royal Road fodder)
Arrow: +10% reduced weaponskill speed and cast time (essentially Haste)
Bole: +20% damage taken reduced (I believe it used to be +10%, but was buffed sometime in HW)
Ewer: MP restoration
Spire: TP restoration
The reason the old card system didn’t work was because there was never any choice between damage and utility: damage was always superior in terms of numbers, especially when more and more jobs began to gain mitigative utility or support utility that could make things like Bole, Spire, and Ewer less valuable (old Refresh/Tactician and Palisade to name a couple). Allowing a dual-Spread that let ASTs hold 1 utility card and 1 damage card could have alleviated this in a sense, because now Bole and Ewer would not have to directly compete with Balance and Spear. SB AST didn’t have the best mana economy, and relied heavily on BRD/MCH’s Refresh and optimized Lightspeed and Lucid + old Celestial Opposition to keep from bottoming out—so Ewer was favorable for them in a pinch. Holding a Bole for a tank could also be beneficial, especially in dungeon settings where a tank is mass-pulling. The most useless card was arguably Spire, since TP was not a concern for the most part; but it could have been changed to something else. Arrow could also be controversial, as there were only a handful of jobs that wanted Haste (BLM, for example), and tons that did not (NIN, MCH, PLD, to give a few).
That said, something would have had to be done with Balance, since nothing can compete with flat damage increases. There was a lot of talk about making it increased Direct Hit back in the day, which could have worked to an extent. I think the problem would have likely just shifted to Spear being king, since critical hit has always been the best substat. Spire would have had to be changed as well, since TP was removed after SB. The developers either didn’t want to try and configure a way to maintain card variety or they thought they were doing what ASTs wanted when they made everything crappier Balances. It’s hard to say what they were thinking. I can sympathize to an extent because AST has always been a balancing nightmare, but I would rather the card flavor over the neutered system we have now. Because each individual card clings to staying relevant: be it by Seals or Astrodyne. Remove those two aspects of the cards, and there’s no reason just to not have one.
There likely is no way for the speedkillers to not value the “best” card of the 6. I realize that. Minor Arcana suffers from that now with it being “Get Lord, be happy” or “Get Lady, be sad”. But I do think that the card flavor gave AST a certain charm, and its something that I miss. Even as one who has optimized jobs in the past. I’m not a speedkiller, but I do like to dabble in optimization. I wish that the old system could have been salvaged in some way. I miss the flavor, and I miss the Royal Road aspects of it. I much preferred preparing Expanded Royal Roads over just single-target buffs.
In terms of healers, it’s quite literally the developers catering to the lowest common denominator. If they think that baby healers are going to be stressed out having to deal with handing mitigation, then they won’t give healers mitigation (or other support skills). Same thing with increasing the amount of damage healers have to actually heal: they think it will stress out baby healers, therefore it cannot and will not be a thing. There is also the aspect of design philosophy: WHMs have asked repeatedly for a raid buff like Bravery to at least make the job competitive with AST, but the developers are stuck in their philosophy that WHM is the “pure, burst/raw healing healer”, and refuse to let it be anything else. And we can all see how well that is currently working out for it. And how well it’s worked in the past, since AST has utterly annihilated WHM very thoroughly since HW (with the exception of early ShB when AST was bordering on HW-launch levels of bad).Now this generally will revolve around people accepting FFXIV's current combat design or not but its the reason for me to believe on their simplification of jobs to be more focused on dealing damage but with no overly complex systems to supplement them. Its why current healer doesnt feel that fun, the lack of having useful supports doesnt cater to its "dps design". Same thing with tanks where they may feel like a second rate dps; because of the lack of having to manage enmity all the time. Although at least with tanks, they are given a set amount of tools for dealing damage and for mitigation but with healers all they got is their various healing abilities but without that robust toolkit for contributing damage.