I totally get that fan demands can ruin a series entirely. I was part of a fandom where that happened and it was the worst.
But fans asking for more screentime for Kairi and for her to be in the action... I don't think that's something that could ruin the whole series by itself. How do you take the request "Hey, can Kairi be cool and help with the fight and stuff?" and turn it into.... that?
Nomura seems to have no problem listening to fans when it comes to Axel fanservice among other things. I mean Axel got put down a bit, but the narrative still had him come out as a better character in end. I can't say that Kairi came out looking better after everything that happened. There were times during the finale that I completely forgot she existed.
Of course, yet it's exactly this "asking for screentime" thing that Nomura can't obviously cater to since there are many fans who want more screentime for characters
other than Kairi.
Like, correct me if I'm on a wrong chain of thought here, but when I look at Naminé, Xion, Roxas or Terra their screentime is even worse than Kairi's, with Naminé standing out as the worst.
The Axel fanservice I'm putting more into the bucket of several staff members also being vivid Axel fans. That he even survived the KH 2 Prologue was largely due to Nomura's staff rather than fan intervention.
Indeed, I agree that Kairi doesn't look any better than before and it is certainly a waste, but exactly because it is done like this is why I think that Nomura either does not care about or is not too privy about details what the fans, especially in the western world, want. I mean, you just need to look across several social media or fan sites and you'll notice how different the demands and expectations are so even if Nomura knew, which of these should he give attention to?
Perhaps that was the intention? The other characters are largely just bystanders, but there are several scenes during the finale where all characters supposed to be the "central cast" are standing together with the exception of Kairi and Naminé, yet the scenes are shot as if they're intended that way.
I don't consider these things "fan demands." More like things that have been common to the series that were inexplicably omitted and deserve criticism. If you think the complaints over those are tantrums, you have no idea what a real fuss looks like.
It doesn't really matter how you name all of these, it's literally the same thing as old school-Final Fantasy fans getting up in arms everytime a FF title doesn't have a controllable airship, explorable world map or turn-based battle system and those to resemble tantrums in several cases, especially when you get people devaluing the entire thing because a few of their expectations are not met.
I've already voiced my dissatisfaction with several parts of KH III that aren't done well including the FF cameos as well, yet I do not expect that just because something is
considered to be a staple by the fanbase that it has to be incorporated everytime.