It's hard to go to bat for any of these choices without knowing where they actually will end up. That being said, I actually think it's better character writing for Kairi to have such a strong negative reaction to being killed than if she had just shrugged it off and jumped headfirst in with Riku lol. Like it makes sense that following that she would actually still have some doubts (especially with Sora not there anymore) and her desire to be with Sora and Riku in MelMem is less "im going to train so the three of us can adventure together" (kh3 repeat) it's "i want to train so that i am an equal to you and sora". I think Kairi in microcosm might be going on a similar journey to Sora, where she needs to find (and believe) in her own inner strength.
Like we do have to consider Kairi being very confident "this time i'll keep you safe" only for her to both get struck down, AND lead to Sora dying, would reasonably shake someone's prior (untested) confidence a bit!
And of course, always bears repeating, that Kairi staying in the Realm of Light actually is likely to lead her to be more important to the story as that's where the Antagonists actually are.