I've been thinking more about this, and I just had to marvel at how miserable of an experience it'd be to watch this ending out of context without any of the buildup before it. Starting with the leak is almost definitely the worst way to start the series. It certainly makes sense that starting at the end is a bad idea, but this one in particular I think is uniquely bad.
Like, even the confused parents bringing their kids to see it in the theater will at least get to see Episode 8, which has a lot more interesting stuff going on in terms of visuals and action, and has a much more initially interesting premise: Episode 8 is about an insane computer god who used to be nice losing his shit and turning evil, and a bunch of colorful cartoon characters have to stop him. The Episode 9 Leak is about an ambiguously gay or bi emo rabbit boy having an emotional breakdown and a midget clown girl who doesn't do clown stuff going into his brain to try to stop him from dying.
The leak (theoretically) only works if you have buildup beforehand. I'm gonna go over how someone who actually likes the show might see it vs someone who has no idea what's going on:
If you don't spend an entire season with the bright and saccharine circus world and see how broken and insane it is under the surface, you don't feel the impact of seeing it all dark, dead and lifeless, mixed with the bittersweet knowledge that the characters who were imprisoned by it are finally free and might have a chance to rebuild it into something better.
If you start with the leak, you just see a spooky abandoned circus that you don't have a reason to care about. It might as well be the Five Nights at Freddy's Pizzeria but without the spooky animatronics and hilarious phone guy.
The appeal of Jax as a character is that he starts off as a douchey, mischievous young punk who acts all cool and also has a scam-artist vibe to him, but slowly as the show progresses you start to see him act more casual and friendly, and then you start to find out his douchey behavior is just a facade to mask deep emotional insecurities and guilt, and slowly you watch that facade crumble as he psychologically deteriorates. It's (to me) an interesting character study on what can make someone act like a jackass and what happens when their coping mechanisms start to fail. You learn he has all sorts of fucked up secrets he's repressing and you become fascinated with finding out the mystery behind them.
If you start with the leak, Jax is a loudmoth, whiny, douchey, emo femboy rabbit right from the start who just complains, says horrible things to people, runs away to cry in the dark, and then mutates into a fucked up blob monster for no discernable reason. Then you spend the next 40 minutes learning about all his baggage and all the weird shit he thinks with absolutely no context for why any of it is important. The Ribbit scenes are some of the only stuff from the leak I'd actually call "great" but again, if you don't have the buildup where you see her sadly looking at pictures of her and getting suddenly uncomfortable when he sees her old room, you don't really have much reason to care that they got in a fight. It's not even like his has any agency in his own downfall because Abstraction is just a thing that happens when you "lose hope." So it's literally "he got so sad he fucking died."
Pomni's appeal is that she starts off as a clueless, helpless and totally confused newcomer to the circus but as she slowly grows closer to everyone, she becomes the change agent that actually manages to break the status quo and find the answers everyone's been missing. Her relationship with Jax becomes a focal point because it represents the part of her that seeks to take action, save everyone and fix the problems that everyone else has already given up on. It represents the payoff for her character arc, and how she's gone from a frightened pushover to the hero of the show.
If you start with the leak, Pomni's just some weird midget clown girl who wants to rescue her emo femboy rabbit boyfriend from literally dying of depression, and again, you don't really have a reason to care about him because he's just been a whiny dickhead the whole time. So the main emotional crux of the "movie" is totally gone.
I'm not saying the standard for a good story ending should be that you should be able to remove everything that came before it and have it still feel like a complete and satisfying story on its own. That would be ridiculous. Hell, I'm not even saying the decision to make the ending more interpersonal and character-focused is a bad idea, because there's plenty of great stories that do exactly that.
What I *am* saying is that if you aren't already invested into the show, I can't see this finale appealing much to you. And when many of the people who *are* heavily invested think it's disappointing... hoo boy...