A healer in Sastasha during Heavensward had actually more to do than a healer in a lvl90 Endwalker dungeon.
Very true, but it had to do with only being able to spam Cure whereas at higher levels you could either apply regens or use off globals if needed (even in Heavensward). Cleric Stance also gave them some risk in that.

Tanks
Tanks were always mostly the same. They are just even more the same now. I mean, they always had a Rampart-like mit, a 30-40% mit, a short-CD mit, a Reprisal-like mit (but this was damage type specific attached to their combo), at least a 1-2-3 combo or two, an invuln, a stun, an enmity generator, a ranged enmity attack...

Many people even in Heavensward were "omnitanks", it just took more skill to be such, but not that much more. It's just that now you can be an omnitank even if you are a completely new player because they play so similar that the differences don't need much or any explaining.

healers are all the same with different skins.
Again they kinda were even in Heavensward. They all had a Cure 1, a Cure 2, Esuna, Raise, an ability heal, an aoe, a single target attack, a dot, a regen and party regen, solo and party shield (actually, WHM had their version of adlo/succor which was Stoneskin and Stoneskin II). And of course, AST could switch between pure and shield healer, so all three of the healers could be pure or shield healers.

The differences were arguably SCH having more dots and ACN attacks but arguably WHM had a decent amount by Stormblood. The autoheal on the faerie was sorta unique but effectively just a permanent regen and WHM/AST could apply regens easily.

The only truly unique thing was AST's card system and it still has that, although it is not as fun as it used to be without a Lord/Lady or Royal Road system for bad cards.

Most differences between jobs were just annoyances ie. the knockback on Fluid Aura or physical-only or magic-only mit.

Everybody and their mothers now have support defensive skills and party heals.
True. Melee DPS can heal themselves with Second Wind and Bloodbath. I think that is only useful to an extent but it can be nice when a tank dies. I was tanking a dungeon boss on Ninja the other day, using Bloodbath. But there is argument to be made that DPS shouldn't be able to heal except for identity reasons like Red Mage.

Although I love my independence as a tank, I had long accepted that I usually would not survive alone as a tank in Heavensward. For example, when I was the last surviving tank in A11 normal, I would just get killed despite spamming Clemency. In the modern day, I'd most likely survive and solo it because of how the game has changed. Clemency could get you through in a dungeon if you were really good, but not usually in 8-person content. Now you can legit finish off an extreme or savage boss as a tank when everyone else has wiped.

What is even the point of ranged physical DPS now?
The truth is, they have gone in the direction of: the point is animations and aesthetic preference, rather than functional difference. Since obviously just BLM has significant cast times now.

There is little to no player agency left, either in job rotations or party synergy.
I think there is room for some more player agency, but I do understand why they went in this direction. There was a genuinly massive gap between good tanks and not-good tanks when we had stances and enmity combos, for example. There were genuine balance issues occuring with damage types. And mitigation types ie. physical-only and magic-only was just annoying. The result of homogenizing physical and magic mit is, well, SE being told they are homogenizing the jobs.

So I understand and agree with a lot of the changes over the years, but I think keeping all these improvements in mind, they could still create a little more agency than they have now. For example:
  • Lending other players MP is alright.
  • Not allowing tanks to survive alone so easily without a healer (shouldn't be impossible, but shouldn't be easy, like Heavensward really).
  • DPS shouldn't be able to heal themselves.
  • AST cards should bring back either Lord/Lady or Royal Road for bad card RNG.
  • Bosses should stop re-centering. It creates as many problems as it solves.
  • Boss hitboxes being big makes sense but they don't need to be THIS big.
  • Make savage mechanics for each role unique again. Tanks used to genuinly do completely different stuff than the other roles but often it is just the same mechanics for everyone now. I can often tell a person who's cleared on another role that it's basically the same as a tank except the tank buster...
  • I also like DRG's current rotation and PLD's former rotation. The new PLD rotation and likely upcoming DRG changes are just gonna make me miss the long, structured and varied rotations they used to have. I don't even care if they are sub-optimal choices because of it. That has never stopped people who just enjoy a job from playing it anyway, it's only stopped DPS-driven players. As a plus, I do enjoy RPR (which is what DRG will likely become a clone of), but part of the reason to play DRG now is how it plays differently.
Everything is hard funneled into fight mechanics and rehearsal and playing a perfect choreography.
Game was always like that, especially since Heavensward when they really became interested in brain-training mechanics rather than RNG.