I look at webpages for various PhD programs and I look and I say "I really can't." I can't do three years or "4+n" years of voluntary prison time. I can't spend two years on ONE problem while life passes by. Not just won't, physically can not.
I can imagine circumstances under which I might - e.g. a part time thing from work, I'm old and powerful with nothing better to do, the whole thing takes two years or less, or maybe the environment has good distractions...
Most recently (today), I considered PhD programs in Statistics, which it occurred to me is the "easy" option: learn some very easy math, then find some majorly large data-sets and say some insightful things about that system - at least this is reasonably well-defined. But to look at the program websites, they've gone and made it about discovering theories about statistics itself. That's alright until you remember that you'll have supervisors looking over your shoulder - they'll be the judges, not you.
I wonder how my students survive...the endless butt-in-seat hours for instance. Or their know-it-all lecturer yapping on about math structures that may or may not lead to progress. I don't know how the many with poor grades survive the hit on their self-esteem. I wish I could rearrange the world so that they'd be doing what they love to do. I include a few things in class to improve the situation: open-ended play with software, connections with culture and development, discussion always about why we are going to learn this, etc
I "survived", actually loved studying engineering in school, because I was very curious about Engineering Systems and Applied Math. I study a lot even now - for no reason at all other than the desire to know. But many engineering students want to be engineers (designing, repairing, team-working, fabricating, inventing) or are undecided. Few want to be analysts when they grow up, and that's what all this theoretical training is most suited to. This misalignment is a waste of time for many people.