Do you regard the non-numbered games as "spinoffs"? And if not, could you respect someone who regarded them as such?
To me, it depends on the game. It's all about how much the supposed "spin off" affects the overall KH story.
Re: Coded and 358/2 Days are spin offs in my book. Basically because their stories don't affect too much to the main plot of KH. And even when they do, the changes or aditions can't be mostly ignored in the grand scheme of things. For example, if not for KH III wanting to bring everyone back, you could perfectly play through the entire series without knowing who Xion is and you would understand the main plot perfectly fine.
Now, Chain of Memories, Birth By Sleep and Dream Drip Distance... I can't consider them spin offs. Because their plots are essential to comprehend the KH story. Skip one of them and you will be left with tons of doubts and questions when you play one of the numbered games. They are too integral to the franchise for me to see them as spin offs.
As for your second question... why wouldn't I? You can think whathever you want. As they say in my country: opinions are like asses. We all have one xD
What I mean is that I can perfectly deal with the fact that you consider them spin offs
I mean, I figured you would be, but there was a thread about KH3 DLC recently that got very heated and multiple users came after me for multiple months because they were upset that I was calling them spinoffs. My original point in my first comment in that thread was something along the lines of that I thought there were too many spinoffs and didn't like having to wait 13 years for KH3, and almost everybody was royally pissed that I felt that we had gone 13 years without a proper Kingdom Hearts game, because they considered the games I call spinoffs, i.e. all the non-numbered games, to be enough to hold them over, while I felt there was a story element that I was waiting for the conclusion of that I knew I wouldn't be getting until 3, and that the games I call spinoffs only served to complicate things further.
I consider them spinoffs because none of them play like the numbered games, and the numbered games all play basically the same, and the numbered games are where the plot you actually came to see happens, while the non-numbered ones are either mostly irrelevant to that plot like Coded and 358/2 Days, or incredibly relevant because they're full of complicated details but they center around stuff other than what I came for. DDD is the worst in this regard, in my opinion, because it's about some test that really doesn't need to have a game dedicated to it, and some ridiculous younger time clone of Xehanort appears, and yes it's all very important, but damn it, if they'd just written things differently, we could have gotten straight to Kingdom Hearts 3! I mostly agree with your original post regarding how I'd take the series, but personally, I liked Aqua, and while the rewriting would probably have to start with BBS, I'd like to keep her character somehow. The concept behind the Mark of Mastery is interesting, but again, we don't need a whole game about that, so no need for DDD.
That said, the series is what it is now and I actually do like the spinoffs, especially 358/2 Days, but I just feel the series could have been presented better in its story. I wish they had made 1, then immediately made 2 with a bit more at the beginning to explain the events of CoM, then have CoM as an optional game for hardcore fans to play who want to know more about what went down, the 358/2 Days for Roxas fans who want to know more about his story, then straight into 3, with 3 written so that BBS, DDD, etc. wouldn't be necessary. I bet if you did that, and made the spinoffs for PS3 so as to help start porting the assets needed for 3 over to PS3, you could have had KH3 for PS3 in 2008 or 2009, just 4 or 5 years after KH2 instead of 13. Then you could follow KH3 up with something akin to BBS, a prequel for people curious about how the Kingdom Hearts world got the way it was, diving deep into the lore for dedicated fans. Maybe multiple prequel games, one for the more recent past setting up the events of the Xehanort Saga, and one for the distant past like that mobile gacha game, except an actual game instead with no stupid gacha elements. Coded I'd discard entirely and the Mark of Mastery would be part of 3, not its own game, so after that we'd be able to have KH4 ready for PS4's launch or at least the holiday after launch. With 2 games discarded, there'd be time to make KH5 by now, honestly. Doing it this way would keep more people interested in Kingdom Hearts for longer, which would mean a larger fanbase, and with the games all being for PS2,3, and 4, it would be much easier to follow along with the series for hardcore fans as well. Everybody wins. But instead we're in this timeline where the story is a mess and certain KH fans try to gatekeep me for daring to envision a better one.