I tried to summarize my path after finishing my Ph.D. in Physics. What was in my mind, and the things that made me focus on a niche that was obviously under-served. I had no tools to think about markets, value, money, but I think I managed to learn quite a great deal in a short time, and wondered who else out there is following a similar path.
Isn't then the PhD itself partly a failure, in the sense of wasted energy/time to obtain the final Certificate, but not using it at all?
What does it mean for you to 'use a PhD'?
Stay relevant in the field of the PhD?
A Phd takes n years. A person could spend n years figuring out what they wanted to do without getting a Phd. The OP has spent n years figuring it out and has a Phd. Plus the habits and skills necessary to earn a Phd that are independent of domain knowledge. Few people have what that takes.
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?
you should
Nobody should unless they choose to. Not me, not the OP, or even “you”. The title is clear.