I would like to respond to some of your points though.
I don't understand how this is unique to osu!. It is pretty normal to shit on and make fun of games you enjoy. It is much easier to find flaws in things that you are passionate about.
I think it's an issue of degrees and osu! seems to hate on itself more than other communities from my perspective. Higher occurrences of people telling others to quit the game and get out while they can, etc. I've been in the League Community and it's the same there with people hating on the state of the game, game mechanics and especially hating on Riot, who creates the game, but in osu! it feels like there is more of a disdain. This could be a matter of perspective, though.
Sure, a lot of players start playing because they saw some super high level play. However, those players usually stop playing after a few hours unless they actually find it "fun" to play. New players often quickly realize that it requires a shitton of effort and dedication to reach the level of top players and without anything other than the "high" of cool plays to keep them going, they will probably just quit playing.
They definitely need to find it fun, but I think there are multiple motivations that go into playing the game. You can have a casual and competitive player that both find the game fun, but one of them is also driven by competition IN ADDITION to fun. You see this in all kinds of games.
Why? Golf can be competitive. Golf is a solo sport. It is also possible to obsess about improving at something without it being competitive.
Honestly, many people still don't understand the appeal of something like golf because it's so solitary. I think there are many people that could never really understand the simple appeal of seeing yourself improve (understand in the sense that they themselves don't care about it) at a game that has no other relevance in their lives.
In osu! the level of "solo" is even more intense. In bowling or golf you are alone, but at the highest level you are often in a game with someone else. In osu! you are always alone even when making your greatest plays. Your "opponent" could be asleep.
I'm a little off base with how I described this one. What I mean is that there is a perception that people taking the game seriously are misguided or strange.
You are putting waaaay too much interpretation into "play more". Playing more is the most important thing you can do if you want to get better. There is of course much more nuance to just playing more, but too many new players are obsessed with "a secret method to improve without much effort" and spends all their time asking people HOW to improve instead of playing. I believe that "play more" is a response used kinda mockingly to say "Hey, you are asking way too many unnecessary questions without actually putting effort into actually playing the game"
You have some interesting view on the osu! culture, but I disagree with you on many points. I think you are looking a bit too much into it and coming up with explanations which are not really supported with good arguments
I agree that in individual cases it is used jokingly, but I think as a whole it persists because of how people view this game (as having little depth and that there isn't anything to be discussed).