Leia being around was a key part of Kylo’s redemption, and they specifically tied her dying to his redemption. It seems pretty apparent to me they only wrote Kylo’s arc (which essentially superseded Rey’s arc in terms of importance) the way they did because they worked backwards from what would redeem him, and they decided his mother’s choice to kill herself by reaching out to him during that fight would be the catalyst.
So they kept her active in the plot by recycling footage and writing dialog to mad-lib their way to the end of the 2nd act, specifically because they needed that presence to register just enough for Kylo’s redemption to pay off as intended.
If anything, Terrio’s answer when he was trying to explain why Rose Tico got so minimized seemed to hint that their initial “We’re not going to CG her” was an out and out lie, (it kind of was already, considering that training scene) and I wouldn’t be surprised if Han’s appearance was similar to how Superman’s mother is who showed up in Superman II once Marlon Brando was cut out. “Well, we can’t CG Carrie for this scene, let’s see if Harrison will do it instead.”
So even setting aside the fact her scenes aren’t particularly good (and it’s fairly obvious she’s not really acting with anyone in the scene, and nobody in the scene is really acting with her) the initial decision to make her self-sacrifice key to Kylo’s redemption, to justify repurposing a bunch of deleted scenes, is what hemmed them in.
It’s why the movie is so plot-focused at the expense of any real feeling or thematic coherence. They approached it like a puzzle first and foremost instead of really exploring what they had to work with at the end of The Last Jedi and building from there. If your primary story question (and they kept saying keeping Carrie is where they actually started) isn’t “Where do I want my characters to go from here” but “How do I repurpose deleted scenes so they’re so vital to our endgame that you can’t remove them,” you’re starting from a pretty mercenary spot, even if you’re doing so with the best, most honorable intentions.