@AuraObscura - A PhD is a research credential. The purpose of getting a PhD is to train to be able to perform original research and in turn, to make original contributions to the knowledge and state of the art in one’s particular field. If you are truly interested in doing original research in some area of mechanical engineering then that is the reason to pursue a PhD. That said, the vast majority of PhD holders aren’t making cutting edge innovations that completely change the landscape of a technology. In my 25 year career with Bell Labs - which was the largest group of PhDs under one corporate roof- with eight Nobel Prizes to its credit - only a small subset of those PhDs were members of the fundamental research staff doing anything that may have become groundbreaking contributions to technology. Of course, some did - the transistor, laser, microprocessor, communications satellite, discovery of the cosmic microwave background (the present-day remnant of the Big Bang), but for every one of those there were thousands that did not. Many of the PhDs (and engineers with BS and MS) did day to day engineering work to develop the products that kept the former Bell System the world leader in communications.
If you want the chance to produce new technology or make a lasting contribution as you describe, that is a good reason to pursue a PhD. If you get a job after finishing with an organization that has the means of producing such lasting contributions (such as one similar to the old Bell Labs, a university, a national laboratory, one of the major corporations that does fundamental research such as IBM, Google, etc.) so much the better. There have been lasting contributions made by non-PhDs doing what you might regard as ordinary engineering work also. Innovation is not limited to PhDs.
As you also point out, there is a significant opportunity cost. If you go back to school full time for a PhD, you are looking at 4-6 years of no income (other than stipends). You will need a health insurance policy for you and your family (most schools include full time students in their group health plans, fortunately). Economically, you will do far better initially by doing engineering work in industry. Some employers will sponsor you to get a PhD either part time or full time. Bell Labs used to do that, when I was working I started the PhD coursework part time. They gave some time off during the week to attend classes (this is uncommon today, however). In the last year of my research and dissertation, I had a doctoral fellowship from the Labs that allowed me to complete the work full time while still receiving a partial salary. For me, that worked out well.
A friend of mine was a chemist with a major drug company. He had a BS and MS in organic chemistry. He decided that a PhD would help him move into the fundamental research area in his company. He packed up his wife and kids and moved to Illinois, and got his PhD in organic chemistry full time at U. of Illinois after 5 1/2 years - he was 26 at the time (that seemed like an unusually long time for a full time effort though). It was a hardship for him to support his family on the stipend he got for assisting his professor with his research, but as with any investment, he sacrificed in the present for a reward in the future. He is now a department head in the research division of that drug company. While a PhD isn’t a guarantee of employment - there is a not insignificant unemployment rate of PhDs actually - for those jobs that actually require the PhD-enabled research skills it is definitely an important qualification.
If you can deal with the opportunity cost and the hard work needed for a PhD, and your objective is to do original research and produce significant innovation then you have the right mindset. I wish you the best!
Michael, Ph.D., P.E., Senior Member IEEE, Consulting Aerospace Engineer