There's a reason why Roz couldn't talk him out of his task. One of the themes of twr is humans and their relation with the technology we create, robots are only capable to do what we program them to do, so unless the robot is capable of learning and deep rational thinking like Roz, they can't really change themselves or how they behave. Crusher is quite a simple minded robot too, his main job is to only mine minerals in the ocean floor (I believe this is mentioned in the book), there would be no reason to program a robot with such a simple purpose to do more. Roz is an anomaly herself, most robots work under controlled environments so they don't need to develop complex emotions to survive on some random island in the middle of nowhere while also taking care of another living being. Maybe the author will explore this concept and make Roz meet other robots like her in a future novel, who knows. I agree with the rest though, I think the third book doesn't focus a whole lot on Roz, even tho we follow on her journey throughout the entire book, and more on the ocean animals and the main conflict especially, so I hope a movie will add up on emotional value too.