Mokou's life has revolved around hating and killing Kaguya for so long that she literally doesn't know how to exist without her in her life. That's why she freaks out so much at the mere prospect that Kaguya might leave. If Kaguya ever did leave, it's likely that unless someone like Keine could pull her out of her funk and give her something to live for, she might literally sit in her shack in the Bamboo Forest for thousands of years.