In contrast, universities and their faculty absolutely need a large supply of low-wage teaching and research assistants to function today.

The students who did their PhD the quickest in my program did exactly that. The students who failed or took longer usually slacked off, skipped days, focus on other stuff, etc etc etc. I know one person who disappeared for weeks and then quit. Thing is, he could still have come back and resumed his PhD even after that disappearance.

You could argue that it’s difficult for students to plan and do work on a consistent basis while having full academic freedom and very little direction (my advisor when I joined with almost no research experience told me to “read papers” and then we’ll discuss what to do after you do; like what does read papers mean anyway; I had no idea at the time but I learned by asking fellow students). But it’s not something that you get lucky in. It’s something that you keep at it until you finish it.