I don't understand what you feel you're arguing here. The poster is correct. The reason leveling and gearing exist is for the player to feel a sense of progression, becoming more powerful. Levels and gear is the RPG equivalent of mastering your art. It's been that way since the days of tabletop RPGs, and I don't see how anyone can claim D&D is a single player game.
If you must become stronger to take on the next challenge, then inevitably the previous challenge must effectively be easier. If the previous challenge you overcame isn't easier now, then the next challenge is effectively moot, you've gained no progression.
This isn't a single player or multiplayer situation, it's how a real world concept is translated into a video game. If every player is supposed to always have the same difficulty at all times, there's absolutely no reason for levels or gear at all. The SE team could effectively balance every single encounter around your base stats and damage of your abilities, change up the encounter mechanics, and just make all gear glam.
Having said that, I do understand the argument for say a level 50 player not seeing the whole fight, because gear and higher level players make the content too easy to blast through. At level 50, a level 50 encounter should be hard, at level 90, a level 50 encounter should be a joke. But how do you balance something in such a way that a new player enjoys the complete fight as it was intended, without making the player who's already completed that challenge feel like they're progression wasn't meaningless?
And fun is subjective. I frequently have fun performing "braindead content" as you say, because after a long day of non-braindead stressful work, I want to sit back and relax with my entertainment. Perhaps on a weekend where I have alot more free time, I might feel an inclination for something more challenging.
What can SE do to keep higher level players filling the queues, while also giving lower level players a decent challenge? I don't know.
But as it stands, you do have the tools to experience harder casual content for yourself if that's your wish. Wear crappy gear, find the duty you want to do, set it to MINE on PF and play with people who have the same mindset as you.