That's literally the point of the provision - to renegotiate the rights after the market value is proven.
For a book, it can be a tool for a writer to get their work away from a publisher they don't like and go elsewhere. In the case of a movie though, the only rights it gives them is that of the script they wrote - nothing else.
If they took the rights elsewhere, it would just be then taking their script to a new studio to get it adapted anew. If that studio wanted all the other parts, they'd still need to get them off Disney.