Hyperbole; unhelpful mockery.
Accurate in parts, but with some very misleading text in the middle.
The goal is not to give the players a good game. The goal is to enjoy a good game with the players.
The DM is not selling a book, the DM is not selling a video-game, the DM is not selling an interactive dining experience. The DM is playing the game with the PCs.
That said, it's accurate that agency is one tool with which a DM can improve a game.
The thread is about one aspect of fun.
If you fail at this aspect, there will be less fun.
Improving the Agency aspect of a game is not somehow in competition with improving the fun. It's one part of how to improve the fun.
When you fail, would you prefer to be able to improve your game?
Or would you prefer to have no options for improvement?
It sounds like you're saying: "I don't need that tool right now, therefore there's no point in anybody ever using that tool."
Gotta say, that's about the least-constructive attitude I've seen here.
"Because illusionism sometimes works, therefore real agency is not important."
You're saying something very strange here.
Overall, it seems like you have one specific truth which is accurate ("Agency is a tool") but you don't understand the value nor the limitations of that truth.