One of the most wild things about these types is that they are so broken over the thought of her going back as Rye. They don't care as much about the woman behind Yuko, otherwise they could just follow her to Rye if that happens. They care more about the Yuko character, even though due to the nature of vtubing, Yuko is essentially an extension of the woman behind Yuko. An exaggerated extension, sure, but an extension none-the-less.
I imagine, more than anything, it just comes down to being a hyper coomer who is worried about losing that coomer content with the current avatar, or the possibility of her maybe moving away from that sort of content if she goes back to Rye. Either way, it's pathetic.