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.
Again, clearly you still haven't read the definition of spinoff. And yes, average viewers can understand Avengers without watching the rest of the MCU movies. They just won't be as invested in the characters if they don't watch the other movies. You don't have to watch the Iron Man movies to get Iron Man. He's Batman, but with robotics genius instead of self-taught martial arts, and he makes his own gadgets. You don't have to watch The Incredible Hulk to get the Hulk. He's just "Hulk Smash!" and that's about it. Radiation or something made him that way, now he's strong. Hawk Eye and Black Widow never got movies yet that didn't stop people from getting their characters in the movie, they just cared less about those characters. Russian Spy Girl Femme Fatale and Guy With Special Arrows And Uncanny Aim, we get it. Thor is just Thor, the literal god of thunder from Norse mythology, except those gods are aliens and their magic is technology, but you don't really have to think about how it works so whatever. That just leaves Captain America, which is the most self explanatory superhero not called Superman. He's a super soldier from WWII frozen until today, and he's super patriotic themed. They're all very easy to understand, and from there everything you need to know is contained within the Avengers movie. If you like those characters, you can go see the other movies, but you don't need to watch all the superhero movies to watch Avengers.
Again, I've played them all, and I get that the numbered games are difficult to understand without the context of the side-stories, but they're still spin-offs. They fit the most straightforward understanding of what a spin-off is. And I still don't get why you're so worked up about it. I'm guessing you still don't care to share?