It is important to understand education through a degree doesn’t necessarily mean you’d be actually good. I remember being a Human Factors undergrad student and running usability studies with a sample size that’s greater than 100. Now this is typical university crap and doesn’t often reflect practical world. In a practical world, you might run a usability with maybe 10–12 sample size due to budget constraints and business requirements. And you certainly don’t need a ridiculous amount of participants to find major usability issues.
I have moderated usability studies with some of the biggest brands in the world and the experience of running and analyzing is more important than whether you have a PhD or not. I learned how to run research through my senior UX research manager, who I believe holds only a Masters degree.
In conclusion, it depends. What do you want to do with the PhD? Do you want to join your university and become a researcher? If so, by all means do so. If you want to become good in research, then it’s a matter of working for a great UX research company and learn from the best.
There are good and bad parts to this. The good part is you have a PhD and it’s highly respected and no doubt you have a wealth of knowledge. The bad side is a company might not hire you because you have a PhD and they know you’re expecting…well PhD salary.