To what extent will your choice of PhD topic restrict your career?
Very few people are static in their careers, in and out of academia. My PhD was in the area of molecular simulations in chemical engineering. My post doc was with a physicist working in computational materials, then I took a staff position in environmental engineering doing fluid flow and geochemistry studies, I have since moved into renewable energy applications and biomedical, with side work in algorithm development and kinetics.

All of this is to say, any limits on what direction your career can take are placed by the researcher, not the field of study. If you’re not willing to branch out, your choice of jobs will be limited. If you’re willing to explore, the options are as infinite as your curiosity and salesmanship will allow.