I have a CPL, BEng(Hons) in aeronautics, and a PhD in the development of flight test techniques. I make money that would be around "regional captain", in practice I've never worked as a full time professional pilot, although I've also never worked outside of aviation. My job is basically as a senior manager in an aviation research organisation, which is more or less what I've done for the last 8 years. Prior to that, I've worked in academia, airworthiness, and flight testing.
There are jobs for those of us who have chosen to combine academia and professional flying, and they are generally very interesting and reasonably (not necessarily very) well paid. Those jobs aren't however neccessarily very easy to find - I suspect that there are less than 100 people with my broad profile across the UK and USA, and I've probably met about a third of them.
It is useful if you're consider this to remember what a PhD is - it's not like an MSc which is a higher taught degree that emparts professional knowledge, it's basically a research licence. If a CPL is the baseline to be paid to fly an aeroplane, a PhD is a parallel baseline to be paid to conduct independent research. Full professors are the training captains of the research world!
So, to my mind, the reason to do a PhD is because there's a topic you have a passion to research, and you fancy long term being a researcher (presumably joining me in this select little band of people conducting indepedent research in aviation). If you do have that passion, go for it - in fact feel free to get in touch offline as I may be able to help you with finding a university and supervision.
Like most people I massively enjoyed about 75% of my PhD, and hated the last 25% of writing up and editing the thesis. For me it's supported my career since, and I'm very glad that I've done it. However, I wouldn't claim for on moment that it is a route to senior management - it isn't. Nor (despite many people thinking otherwise) is it a mark of substantially greater broad subject knowledge - whilst I'd hope that anybody with a PhD in subject X has an excellent knowledge base in the broader subject, that's not actually what it's about.
If you want a route to management, to be honest, I'd do either a general or specialist MBA, not a PhD. [That said, I have a BEng+PhD and my wife has a BSc+MBA, and I've ended up with quite a bit more management responsibility than her, which is only cherry picking one example, but does show that PhDs can end up in senior management.]
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