What if my professor writes me a negative LOR, in order to keep me working with him?
Writing bad letters of recommendation because of personal disappointment about a student leaving is so disproportionately unethical that I cannot believe that it happens with any significant frequency. It is not what I would be worried about.

In the end, most professors take pride in their students growing up, going out into the world, and being successful. Students are a bit our kids too -- sure, it would be nice if the kids stayed in the neighborhood, could continue to mow the lawn, and come over for dinner twice a week; but if they move out and are successful, we're proud nonetheless and hope they stay in contact. And if there's something we can help for them to be successful (like writing letters of recommendation), then that's what we do!