Mark Taylor, a professor of religion, has observed (in the New York Times) that:"graduate programs in American universities produce a product for which there is no market (candidates for teaching positions that do not exist)…[with] sometimes well over $100,000 in student loans."
This is not really true of the sciences, where the main product is the research that is central to graduate education, research that often leads directly to improvements in healthcare, agriculture, engineering, or environmental quality. Nor do science PhD's usually take on much debt. (If you are applying to grad school in science, and they don't promise you fellowship support or paid teaching opportunities sufficient to meet minimal living expenses, it's either because you are poorly qualified or because the program is poorly funded. Either way, you should reconsider.)
But programs in the sciences do collectively graduate more PhD's than they hire, so a PhD is no guarantee of a faculty position.
Denison goes on to argue for tenure in the sciences, or at least being aware that taking away tenure could have an adverse effect on the scientific output of Universities, which in turn would have an adverse effect on our country, and the world, as a whole.
He may be right. But, he's wrong about the debt. At this point, the debt accumulated isn't from ones graduate program generally, it's from ones undergraduate program. It is VERY easy to graduate with more than $50,000 of debt from your undergrad at a STATE school. Way higher from private schools. Only people with rich Mommies and Daddies (or a willingness to go part time for 6 or more years while working full time) can avoid it.
And besides, tenure is a dying dream in ALL fields. Unless you graduate from one of the top 20 schools in your field, forget about getting a tenured position at a research university. That's a pipe
But, don't worry, the adjunct life ain't bad. And PhD's at a think tanks and research firms don't do bad either. Neither of which are affected by the tenure system in a direct way.