There are lots of careers that benefit from or require a PhD besides being a professor. People with STEM PhDs are found in great concentration in R&D settings. For example, when I worked at NASA JPL probably about 50%+ of the scientists had PhDs.
Outside of STEM activities, it can still be an enriching life experience. You don't incur significant debt. It does have a high opportunity cost, if one's undergraduate degree is lucrative. On the other hand, you get the privilege to spend your time trying to create new knowledge.
Real PhD Issues (and Cons):
1) Research is hard and it requires a massive time investment to create new knowledge, especially when just starting.
2) High levels of impostor syndrome. You will likely meet people much smarter than you, and those that can work 80+ hours per week for years. If you fall into the trap of comparing yourself to them, your mental health will suffer. You have to just work toward solving your problems. As one old friend put it, he felt like a genius when he worked as a software developer, but throughout his PhD he felt like an underachiever.
3) Jealousy of your friends "getting their lives started" (kids, buying a home, etc.). Very hard to have any work-life balance as a PhD student.
4) Don't expect to get rich after a PhD. Do it because you think a career in research will be satisfying.
5) If you want to be a tenure-track professor running a lab, you have to be very flexible with where you live. Being a professor is also very stressful.
6) If you want to teach in college, a PhD is required but the main thing a PhD teaches is research -- not teaching.
7) Clearly assess your ability to get a professorship if you choose to do a podstoc. There is little point in one unless a professorship is your end-goal. Many don't realistically assess their competitiveness for professorships. I wrote a blog post about this about 6 years ago (it needs updating): http://www.chriskanan.com/planning-for-life-after-your-phd/