As much as I agree with OP I think the days of 3.0, tank diversity is long gone. The problem is elitism + poor raid design = dev playing safe. The reason why pld sucked so hard in 3.0 was because most, if not all, the raid fights had magic damage for a tank that exclusively excelled in physical mitigation. I honestly thought that PLD was great in what it did and if it had Clemency at that time things would be very different but the insane self sustain and damage of WAR and magic mitigation of DRK was just too good. FFXIV has been scarred since then and it finally led us to where we are currently with all tanks having the same cds, the same answer to any situation and roughly the same damage output. I put the blame on not just elitists but the Devs as well for lacking foresight.
Just to note some things that tanks lost on the way to Shadowbringers:
-cross class skills (pld used to have raise outside battle via CNJ fyi)
-niche abilties like mercy stroke
-Job exclusive interrupts like DRK low blow procs
-Physical and magical damage exclusive defensive cds
-enemy debuffs like int down or str down (debatable if removal was a good thing or not)
-Cd synergy like Raw Intuition and Awareness
-Threat management, there was a time where you actually had to tank to make sure the dps or healers won't steal hate.
-Tp management (also debatable)
-Job exclusive tank stance (imo they got lazy with DRK tank stance since its just a reskin or PLD)
-Off tank stance tied to gcd or long ogcd (requied thought as using at the wrong time may end up losing boss aggro)
-Stance dancing
-Bloodbath rotation
-Berserk-Infuriate-Inner Release rotation of 3.0
-Utility heavy tank vs damage heavy tank vs magic mitgation tank These used to be PLD, WAR, DRK respectively but now all tanks have an answer for each situation
-Skill based tanking tanks used to have a high skill ceiling that separated the player base (debatable if it needed to be removed completely)
These are just off the top of my head and I'm sure I'm missing more since I've been with the game since 2.0 beta as WAR and been through the highs and lows and currently its a low for me imo.