Generally, I agree with this. It may be confirmation bias, but I feel like there's been a higher instance of people missing and/or ignoring mechanics lately...

As a recent example, I was healing Paglth'an for my Expert Roulette the other week. Everyone zoned in, I didn't notice anything peculiar, expected a standard Expert Roulette run for the day. Tank starts by single pulling packs... which, by itself, is not really a problem, just a little abnormal and slow for what I'm used to in that dungeon at this point in the content lifecycle. It was kind of a problem that he wasn't consistently picking up aggro on every mob, but even that's not the end of the world, necessarily. We get to the first boss... tank does literally zero mechanics after the first round of them. Okay, he's new... I can deal with that too, I explain to drop the lightning circle off on a rod. Tank literally does zero mechanics for the rest of the fight, gets hit by all AoE's and dies twice from the vuln/lighting vuln stacks, despite me explaining 3 times. Second boss, no better - literally hit by every single thing, we wipe once, I explain to get out of the giant laser AoE's into the one safe zone... no response, continues to get hit by everything. I rescue him to safety once, but Rescue's CD is not short enough for me to use it every time. DPS accidentally facepulls the next trash pack while the tank is still in the boss arena... we burn it down, as we are doing so, tank drops group.

Not a word said the entire time, no response to multiple chat attempts to teach the fight, then drops. I'd say maybe it's someone who bought their way to 80 PLD, but he also had a relic sword and shield, so...???? I was very confused. How do you get to the literal last available dungeon of the current game and seem to not know anything about how to execute your role or do basic boss mechanics or even read or respond to chat? Even the lack of response I could chalk up to being a console player, but from the fact that nothing I said impacted performance in any way, it was clear it was not even being read, so... IDK, IDK.

To the OP's point about what are dungeons in Endwalker going to be like if this keeps up... there's always trusts for your story runs of the dungeons, and hopefully there will be enough of our long term players that come back by then that this is less of an issue. I think that is part of the recent problem too - not only an influx of new people, but many longer term (i.e., arguably more knowledgeable/"better" players) are probably taking a break at the moment if they're not grinding relics, because there's not a lot left to do until Endwalker releases, for a lot of them.