Yes, but this is just...SO absurdly wrong. As someone said:
It's like comparing a modern WoW raid to Molten Snore where players were not using all their abilities because they might draw agro or hit the boss' debuff cap. Where Hunters literally just auto-attacked. While the toughest fights back in ARR were tough, there were a lot of borderline tank and spanks or fights that only required 1-3 people to actually know what they were doing.
Look at NIN's rotation at level 50 and compare it to now. One of those looks "dumbed down", and it ain't the EW version. At level 50, NIN's didn't have Ninki. They didn't have Forked/Fleeting. They didn't have Hyosho Ranryu interacting with Kassatsu (indeed, you didn't even need to memorize the Hyoton Mudra combination...). No Ten-Chi-Jin. No Meisui. No Huraijin. No Phantom Kamaitachi off of Bunshin. You had a bunch of 1-2-3, the occasional 1-2-4 (for a different reason, though; the slashing debuff if your party didn't have a WAR, I think it was...), and a bunch of Raiton.
NIN is hardly the only Job we could say this about. BLM now is far more mobile than it was in ARR, but fights also REQUIRE a lot more mobility, and it's a far more technical Job to line that mobility up with boss fight phases. It also has Fire/Blizzard 4, Umbral Hearts, Polyglot stacks, Ley Lines, Sharpcast, Umbral Soul, and Paradox to play around with.
The Tank rotations at the time were borderline nonexistent. PLD had Rage of Halone combo...and that was literally it. There was no Confetior combo. There was no Royal Authority yet. There was no Goring Blade yet. There wasn't even Clemency, though you could cross-class Cure 1 and Stoneskin. PLD had a 1-2-3 combo, Tank stance at level 45 or so, and Flash. FLASH. Flash - and JUST flash - was PLD's "AOE rotation". I guess you could count 1-2 Riot for MP to keep casting more Flash as a "rotation", but it wouldn't past muster in the modern game. People talk about modern PLD post 6.3 being braindead, but it has twice the complexity that ARR PLD had. The only other thing ARR PLD had going for it was Sword Oath stance, but that was something like your attacks doing a bit of extra autoattack damage? It's been a while, so I don't even remember other than it was not compelling and challenging gameplay.
Yeah, you can point to a case like SMN, but the funny part is, modern SMN is about as mechanically complex as SMN at 50 was. The often complaint about modern SMN? "It's like a level 50 Job". SMN in ARR was "apply DoTs, use Aetherflow on Fester, poop out Shadow Flare, spam Ruin until something needs refreshing". Riveting, I'm sure.
DRG? It's more mechanically complex to play now than it was then.
MNK? Less positionals, WAY more mechanical complexity - MNK back then was rotate through your 6 basic abilities. I'm not even sure if they had the Riddle/Fist of <element> abilities back then, but I don't think they did.
BRD? Didn't even have Iron Jaws. And things didn't proc like they do now. Songs didn't work like they do now.
SCH? Less DPS kit now but its healing kit is far more advanced than it was then.
WHM? If you look at it by the numbers, they only have one less damage ability now than they did in ARR. And despite people fantasizing to the contrary, WHM's didn't "Cleric Stance dance" until late in ARR and really into HW. World First clears of content would often not see the WHM in Cleric at all, and sometimes, not even casting a damage spell due to MP constraints. It was Cure 1 Freecure fishing for Cure 2, Medica and Medica 2, Regen, and Stoneskin.
WAR I can't say - because it had those kind of ridiculous MRD cross-class builds,
The only arguments you might have of Jobs at 90 that AREN'T more mechanically complex than they were at 50 in ARR would be WAR, WHM, and SMN, and the latter two of those are debateable.
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Maybe we can look at "average" encounters, but most of the bosses in the EW 24 mans have mechanics comparable in complexity to those in the Crystal Tower raids. For example, in Labyrinth, the first boss was changed (it originally required 6 tanks), but the second boss is kind of just "one party attacks boss, other two parties kill every add they see", big bomb is "each party tanks one of the sub-bosses, which does a tankbuster and a point blank AOE, kill the bombs that spawn, then kill the big bomb once all the subbosses are dead". Behemoth is just "drop meteors outside of his hitbox, kill Iron Giants so they don't destroy your cover, hide behind the meteors when he summons the big one". And the final boss? Don't stand in the VERY apparent AOE telegraphs, kill the single hand add that spans for your party, go to the outside and stand on a pad when he moves to the center to cast his big flare attack, kill adds there, repeat until dead".
None of those require the quick and precise levels of movement of modern 24 mans. None of those mechanics even compare to Halone's 4 hit memory game mechanic, much less everything ELSE in that fight or in that raid.
Some people like going on and on about Orbonne - the 24 man that had to be nerfed a half dozen times because at launch, groups were literally timing out of the instance without a clear - but that was (a) in SB, not ARR and (b) probably the hardest difficulty "casual" content ever released into the game. And the result wasn't players being better, the result was people not clearing and eventually people avoiding or dropping from it so NO ONE ELSE could clear, either.
One of the Coils fights was literally just an extended add phase. And if you exclude stuff that was buggy (Divebomb) or ridiculously unintuitive to the point even people clearing it weren't sure how it worked (Twister), they mechanically aren't some world more away from Savage raids today in a lot of senses. The Extremes back then were also mechanically easier - players were just not as good. Indeed, Extremes now are mechanically a lot more involved than most of the ones back then were, as are the player rotations required to beat Enrages. The reason we have good clear rates is just because the player base has gotten better over the years and gotten used to various mechanics and how to resolve them.
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I'm not sure if the average player skill has gone up, down, or stayed the same - I highly doubt it's gone off the cliff like your absurd "graph" in the OP wants to say - but Jobs have often become much more complex over the years, and encounter design has become more complex as well.
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Basically this: