The point isn't that people can just spend 80 minutes soloing a dungeon. It's that the mechanics are so relaxed that someone can solo the dungeon, even if it takes significantly longer to do so. Obviously any content that doesn't scale to the number of players will inherently go quicker with more people regardless of whether or not more people are required. I did that test in a response to what you had asked, if it was possible to effectively solo casual content like a dungeon as just a healer. Based on what you were responding to, the implication of your statement was that regardless of how easy the content is, you still depend on other players to clear it. But my challenge run as well as that video series I shared prove the opposite. And the thing is, I ran that dungeon synched. You can still clear content unsynched for quest progression, so you can absolutely just over level and solo dungeons in a far more reasonable amount of time anyway.
But Apkallu's point is where I think the most substantial point stands. The standard experience feels like you're going through the same hallway simulator adjacent to 3 other people, not necessarily that you're working with them to reach the end. And niether the mechanics nor the content's lack of need for the role system are have helped that in recent years.
I'm not sure if I understand what you're saying in your second paragraph, but it sounds like you're proposing nihilistic defeatism. "Even if the next expansion changes, the expansion after could just go back to this" So I guess we should just never do anything about the game ever, because even if changes are positive, those positive changes won't matter because they'll inevitably be subverted by worse changes later. Wasn't that literally what the Endwalker Story stood in opposition to?
As far as the topic is concerned, I want to stress that for the most part, there has not been agitation in the face of solo player content prior to Endwalker. Why? It goes back to a post I made a few pages earlier. There's nothing wrong with supporting solo play and fostering an environment that allows solo play to be an option. I would argue that it's a great thing, but it should not come at the cost of the multiplayer aspects of the game, because this is a "massively multiplayer" online RPG. The multiplayer is a part of the point. And Endwalker has felt like an expansion where mutliplayer elements were sacrificed for solo play.