There's alot here to reply to, I hope I clarified at least most of it.
Dungeons and normal trials are always challenging the first time. New mechanics are introduced, between HW and SB and then SB and ShB there's a large leap in difficulty in mechanics. New icons are introduced, new patterns, less telegraphs, more requirement to watch your surroundings or the boss, or know in advance what the ability in the cast bar does.
And alot of these mechanics can feel cheap to new players, because they outright die on their first or second attempt, because it's as if the game expects them to peer into the future.
The challenge is lost on repeated attempts, once a player knows the fight and exactly what they need to do.
You explain it as some personal affront to you and other players who like high-end content. It's not. It's a video game. It's meant to be fun entertainment. Treat it as such. And yes, if you don't like the average PF player and don't want to play with them, then don't. As I said, you have the tools necessary to associate with the types of players you want to play with.
Interesting you should mention this. Because I've seen this with a friend, during my early sprout days. Except, this person wasn't interested in quick queues or rewards, they just wanted to try a healer role. A friend who enjoys games casually, only for fun. Having a tank sit at the entrance of the dungeon and not move on the first wipe, then cast a vote kick, in a level 30 dungeon no less. Without going into details, this is a person who's done far more with their life than most, experienced more, and who outright quit the game, because some veteran player just couldn't be patient, not out of anger, or spite, but because it was just so silly. A veteran player drove away a sprout player who tried to heal their first dungeon. Tell me who the real dead weight was? Tell me again how this is SE's fault? Tell me again how gatekeeping is an invalid argument?
You chose to join this person's PF or join the Duty Finder, whichever case it may be. As such, you chose to play with what you were given. This person does have an obligation to do the best they can to clear, as do you. If said player is standing around doing nothing, you've every right to complain, as I stated before. But their skill level versus yours doesn't matter, worry about your own performance and not about what the other person is doing. At most, you'll lose a few minutes of your day, or, you can exit gracefully.
You may very well be right the a slight increase in difficulty might help players become more accustomed to it. It may force players to improve, it may drive them away.
But, what about the third possibility? The risk in the increase in resentment between good players and casual players, as now the casual players will feel more pressure to perform or be called out, and the good players who will have even more time out of their day wasted, because they'll feel like they need to put in additional effort to carry the bad players?
I'd like to address this point, because on this, it particularly applies to me too. While I've no interest in Savages or Ultimates at all, I've neither the time nor energy to put so much effort into doing those, when it comes to Extremes, I've done the ARR and HW ones sync'ed, while I was still a sprout, and stopped doing them after HW expansion. The reason wasn't the difficulty spikes, or the fear of new mechanics; it was other players. I've completed plenty of single player games on their hardest difficulties. It might take me a bit longer than other players, but I already know I can do it. It's not a fear of difficulty.
It's the types of players who frequent these forums complaining about other players' skill (and I don't mean you or anyone else specifically, just in general). Because, to players who've already completed these many times, such that they became easy, they don't have the patience to work with new players still learning the ropes. No, you're not expected to go in blind, you must lookup the guide. No, you're not allowed to read the tooltips and learn a "good enough" rotation that works for you, you must join The Balance and learn your optimal rotation. No, you can't fail the mechanic more than a few times, you must learn the mechanics and understand them on their timeframe.
There's plenty the game doesn't teach you properly, I've no argument with that. And I'm definitely not against more resources in the game to teach more. But, if it's not in the game, then the game isn't demanding it. If the game isn't demanding it, then I'll not be held hostage to the demands of other players who feel like I should play the game their way. That's why the arguments for gatekeeping come up so much.
The veteran playerbase has a my way or the highway attitude to the harder content, which, I've no issue with in theory, but I'm going to choose the highway. I'll wait for that content to be easy enough for myself and a couple of friends to do it unsync'ed. Because, at the end of the day, that's all it is, just a game. Entertainment. Being good or bad at a video game means nothing in the grand scheme of things.
Many players stay away from harder content because of the attitude other players display, and that's not the game's fault or SE's fault.
I'm not. I came from SWTOR.
This is a bold statement. It assumes your expectations are reasonable to begin with. In my personal opinion, as long as the player is pressing their buttons, trying to be active within the fight, and don't try and intrude on a PF clearly marked for good players, clear parties, farm parties etc., that's reasonable and they're not dead weight and can enjoy the game any way they like.
In many ways, I'm glad at many of SE's measures to avoid toxicity in this game. If the stories I've heard about WoW are true, if FF was anything like it, I would've left the game within a few months. So, whatever SE is doing, seems to working, because I'm still here, 4,5 years later.