Realistically, any game will have people who want to optimize; for some, it's literally the endgame. (It's honestly starting to get that way for me. Well, one of the two endgames; the other is glamour.) It's why the Balance exists here in FFXIV, among other things. Others will want to play their own way.

In a single player game—take Cyberpunk 2077, as an example—that wouldn't matter. The person who wanted to build out V's skill tree such that they were numerically optimal in combat and the person who wants to do stealth and katanas solely because "I think ninjas and stuff are neat" do not affect each other's play in any meaningful way.

In an MMO, though, they do. And if someone's preferred playstyle means a dungeon takes 40 minutes, or a raid involves 5 wipes, versus a quick run? Someone else is gonna be not happy. Because one person's playstyle can hinder another's progress.

Moreover, there's a lot of ability interactions that effect the party overall, not just progression in the abstract; two dancers both hitting Technical at the same time wastes the Technical buff, which is discarding potential damage for the group. Same with a monk never using Brotherhood. A SCH and AST both shielding someone will overwrite the shields (unless the SCH crits and gets Catalyze, which does stack with AST) and be wasted potency. Etc.

And it's particularly pronounced in serious endgame content, because people often get focused on efficiency; if someone is playing in a way that's fun for them but isn't optimal for the group, they absolutely might find themselves feeling pressured to change to a more optimal method.

No matter what the devs do, someone's going to figure out the 'optimal' method and many people are going to prefer those they do content with use that method. (I mean, how many people on the receiving end ever really wanted cards other than Balance most of the time prior to the AST reworking?)

Granted, there are games where there are exceptions; old school Secret World was one, because the deck system literally was so broad that there were easily 10 different ways to build a super-effective deck to do Thing X for any given X. Yeah, there were some decks that were considered 'optimal' in serious content, but people were always finding new weird ways to put stuff together, so folks were more open to alternatives. (My blood/elemental build with passive traits from assault rifle had honestly less than zero justification to work as well as it did... but it was stupid effective as a combat healer, so no one questioned it.)

On the other hand, it was seemingly near impossible to actually balance content in any sane way with such a wide spread, and old-school Secret World also had about 30+ really badly broken ways to do Thing X for any given X. Which was hugely problematic as a combination; with no real guidance on building an effective deck, I know a lot of people who built something 'fun' that got them through Kingsmouth and the Savage Coast, and then when they hit the Blue Mountains and the difficulty ramped up a little bit, their utterly broken builds hit a wall and they felt unable to progress. More than a few left the game rather than try to figure out how to rework their deck. Meanwhile, others had absurdly effective deck builds and just blitzed the entire third zone's content like mowing a lawn.

So diversity in builds/specializations/etc. is not always a good thing, either.