Not everybody finishes their PhD. I believe there are a lot of successful people who started something during their PhD that turned out to be more than just a dissertation. You could simply shift gears and go ahead with the startup, hoping to finish the thesis later (maybe using your startup as a research site or data source?) or you could focus on the PhD project now and monetize it after you graduate. Reflect on this question: do you really want to be a professor, or primarily value the sheepskin?