I've nearly completed a PhD in engineering. A while back I decided that academia is likely a dead end unless you win the lottery. If the adjunct situation won't improve until people stop taking those jobs, I'll do my part. I'd like to periodically teach a class or two, but I would never adjunct as my sole source of income.
It also covers a bunch of human costs, such as the staff handling payroll, grant reporting and other compliance costs around the grant, the campus fire protection and police departments, etc. etc.
To say that the overhead goes to flashy buildings and fat admin paychecks is a bit of a misrepresentation. Those buildings almost always have a major donor and a substantial endowment component. The academic (especially physical and biological science) divisions do actually need to pay for office space, lab space, heat, staff to run it, clean it, make the computers go, etc. If you think my salary was ballooning while building and running everyone’s HPC clusters, I can assure you it wasn’t in any sense other than perhaps slowly deflating over time. Compared to whatever the football coach and president are being paid, there are thousands of people on staff who aren’t getting those fat paychecks.
Try 90% and almost all of it goes to the football coach or the dean’s legal fees.
https://osp.mit.edu/facilities-and-administrative-fa-rates
HMS declared rate is 58% but there are a lot of loopholes (as at other R1s with Institutes)
I took my Ph.D. and joined industry rather than try tenure track lotto.
Back when I was thinking on pathways, I was a fresh faced noob with a few papers to my name competing against senior scientists with decades of publications from the former soviet union. This made the decision much easier.
If you have a partner that could be good or bad depending. Your relationship will be stressed, I have seen several relationships flounder due to the process and pressure, but also I've seen several come through ok, and a couple blossom. Alot of these observations correlate with normal tracks for romantic relationships, life is full of surprises and challenges, but the big fact is that people who have lost lovers and spouses during Ph.D's often blame the Ph.D (in my experience).
If you have (young) kids I strongly recommend that you think really carefully. The difference here is that no matter what happens you have dependants, and you will have to be very lucky not to get compromised by this relationship and the Ph.D process. Something will have to give, and this might mean that your work is devalued or derailed.
Always remember that a Ph.D is a gamble and an indulgence. No one needs one, and no one is guaranteed success. On the other hand it gives you skills and training that I think are very hard to get any other way and for me it was one of the best periods of my life (one of!)
Good luck.