I completely agree that Crystal's has done a great deal to diversify the runway and fashion at large. She's practically been the sole face of body diversity and the plus size movement, and you're right, she has created her own category. I also think it's great that she is getting campaigns and editorials in the mainstream straight size world. And I hope she continues to do so.
However, speaking strictly about the continued use of Crystal to represent the plus size movement (and this is perhaps more directed at the plus size clientele that continue to hire her than Crystal herself) but to use her as the face of a plus size brand right now is kind of ridiculous. Evans UK, for example, starts at a size UK14 (US12) and they continue to use Crystal in their campaigns. She is at the most a size 8 right now. For them to use her when she literally is too small to shop in the store is absurd. I realize that fashion is meant to be aspirational and Crystal is considered both a relatable yet aspirational figure, however to put it bluntly, at the size she is right now she is not relatable to a modern plus size shopper. One that would probably shop at Evans UK. Yet this is, in part, the way she has continued to be marketed. Though, don't take this to mean that I don't think she shouldn't represent the everyday "normal sized" woman, she can and she should. But, think of it this way, you wouldn't cast a size 16 model to represent a straight size brand (that capped out at a size 12) so I don't see why you would cast a size 6/8 model to represent a plus size brand.