Well that's just not what spinoffs means, dude. Sorry. And side stories can have important plot points that inform your understanding of the main story, that doesn't stop them from being side stories. And no, MCU isn't "different sections of the same story". They're different series. You don't have to watch any of them to understand the Avengers series. You don't have to watch any series within the MCU to understand any other series. That's why the MCU does so well.
Also, you undermine your own point by including Shadow the Hedgehog. It's a spinoff, but it includes story points that intertwine with the story points of SA2. Spinoff also aren't junk. Is that why this upsets you so much? Because you think spinoff inherently means junk? Luigi's Mansion, Metroid Prime, Mario Kart, all the Metroidvania-style Castlevanias, Persona, Mega Man X, Donkey Kong Country, for that matter even Super Mario Bros. (also technically a spinoff of Donkey Kong) were all spinoffs that became their own much beloved game series. I'm not trying to undermine some of my favorite Kingdom Hearts games by calling them spinoffs. I'm just saying I wish the presentation was different, and no, not just with a superficial numbering system.
Sorry, you're wrong. You know what the single biggest complaint about KH3 was? People didn't know what was going on because they assumed every other title was just a spin off and not a main entry. They might not be numbered, but they're not spin-offs. They don't just have important plot elements in them, they tell entire sections of the plot that are necessary to understand the majority of Kingdom Hearts 3. And if you think you don't have to watch any of the other MCU movies to understand Avengers... I mean wow, man. To a comic guy, yeah. I can figure it out because there's source material I can reference internally and come to conclusions. But average viewers?
Intertwining with a story and being absolutely integral to the story-telling process are two different things. You think Chain of Memories is a spin-off, but it's a full-fledged game with a second scenario and without it the player would have no idea why the game starts with Sora in stasis. From running through a field to waking up inside a big white sleeping lotus. You think Dream Drop Distance is a spin-off, but without it the player would have absolutely no idea why everyone is hanging out at Yen Sid's Tower and why Xehanort is suddenly everywhere. From reading a letter on the beach to having failed an entire Mastery Exam and losing all of your power after nearly falling to darkness. Shadow the Hedgehog gives additional details on something inconsequential to the plot of SA2. Non-numbered Kingdom Hearts titles are literally the next sequential act in the same plot.
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