Sure, but to earn a PhD you have also to submit and present an amount of publications and even after that, to make the title relevant, you should continue with the subject and the activities.
It depends on whether you judge degrees from a purely utilitaristic view, plus PhD is closer to working than studying. If you work 10 years in a company, and then you write a book, would you wonder what are you doing with the title you earned at your job?
