Coming from the person who calls integral chapters of a story spin-offs just because they lack a number in the title, your words lack a lot of weight. And no, they can't. It's literally why my sister-in-law hated Infinity War- she had no idea who anyone was or why the Infinity Stones were important because she doesn't watch Marvel movies. No matter how much you try to play the "but Batman is Batman, everyone knows Batman" card, Infinity War and Endgame have a plot that you need to watch specific Marvel Movies for. Marvel movies that you could consider spin-offs of Avengers, or at very least spin-offs of Iron Man. Your argument falls flat out of the gate on this one.
And they're not difficult to understand. They're impossible. You're talking about literally 75% of the series prior to the release of Kingdom Hearts 3, and that's not counting Fragmentary Passage. Players wouldn't know who Terra, Aqua, Ventus, Xion, or Eraqus are. Players wouldn't know why Apprentice Xehanort is an old bald dude now. Players wouldn't know why Axel keeps having flashbacks of a girl he doesn't know. Players wouldn't know why Organization members are returning, or why Time Travel is suddenly a thing, or why Sora is so weak, or why Riku and Mickey are going to the Realm of Darkness. Hell, players wouldn't know why they didn't go sooner. The only aspect of "spin-off" that they fit is that they don't have numbers in the titles in a series where literally some of the games have sequential numbering.
We're not talking about the year Daxter spent as an exterminator while he was looking for Jak. We're not talking about the details surrounding Shadow the Hedgehog's inconsequential time spent with Maria. We're not talking about the time everyone in the Mushroom Kingdom stopped what they were doing to have a go-kart race. We're talking about things like how the actual plot of KH3 was set up and explained in another game, and without playing that game you're expected to sit there and say
And the reason it matters is because calling the majority of the series "spin-offs" devalues their perceived impact on the story. If that notion wasn't correct, then guess what? The leading complaint on the game wouldn't be that no one knew what was going on.
Your sister is one person. I have friends that only watched the Avengers movies and liked them and felt that they understood them well enough. My dad also didn't watch the others and picked and choose which ones he wanted to watch. And I've never read the comics, yet I still feel this way. The characters are that self explanatory, their origin stories that irrelevant to the plot of the Avengers movies. So no, my Avengers argument is fine.
I've said before that I love the spinoff games. I'll repeat what I said earlier: "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." To add to that, the numbering system is just a symptom of the larger problem, namely that an artistic decision was made to present the story in a certain way (and not a good way as far as I'm concerned), and the numbering system was made to represent that decision right in the titles. The only advantage to how they did things was that they got to experiment with the battle system a lot by creating spinoffs, and that created some interesting and fun battle systems, though none I would want to replace the main series' system.
And no, simply playing all the spin-offs wouldn't help everyone understand what's going on. It's a running joke even amongst diehard fans that the series is overly convoluted. I thought forevercloud3000 put it pretty well earlier: "Many of the Entries of KH games after 2 feel like Assassin's Creed level of "Spin off but A sequel but not". Those games wore the continuity out way too thin. And the a few elements they did add in are so boisterously convoluted they muddle the experience of KH3 once we finally got there. I am not alone in saying KH3 was a massive disappointment in the story department mostly due to the plot threads added in from all those portable titles."
And I'll ask again, why are you fighting this so hard? You're not going to convince me. I have the definition backing me up, and Kingdom Hearts cited as examples of that definition. That's enough for me. Just let me have my view. You're just like the other gatekeeping people before you. Sorry dude, but I don't have to agree with you to be a Kingdom Hearts fan. So you can be like Keybladewielder and Basil and Nautilus and white knight the series against me the internet meanie, but it doesn't make you guys any more fans of the game than me. I seriously cannot wrap my head around why you're so offended by the fact that I consider the non-numbered games to be spinoffs. Disagree with me if you must, but you're going to have to accept that I disagree with you.