Okay, even so, I must push back stronger on the point you made initially, that professors are looking to exploit free labor and therefore would never cut Ph.D. enrollments in half.
The first notion I would like to dispel is that we are looking for employees of any kind. To be clear, a Ph.D. is closer to an apprenticeship than it is to employment, so benchmarking it against employment leads to all kinds of wrong conclusions about the arrangement.
In fact, we are looking for students, which is to say that when admitted to a Ph.D., students lack the skills necessary to undertake the work, and the expectation is that they will be trained as a matter of course. How this differs from typical on-the-job training is that the training period can last years.
The flip side is that business has adopted the philosophy "fire fast", meaning that that if some employee isn't working out, it's best to just let them go rather than let the issue persist. With a student, the philosophy is reversed; a student who isn't working out should be mentored.
All this training and mentorship does not come cheap. For one, the facilities they use are expensive. For another, faculty time is expensive as well. Students don't appreciate this because the economics are hidden from them, and it often leads to perceptions that they are only paid the small fraction of their compensation that flows into their bank accounts. But to be clear, this training is in fact compensation to the student. It's not given to them at zero cost to the university. I can charge up to $300 per hour to give advice and training as part of my consultancy, but my students as part of my regular employment get carte balance access. They don't ever see an invoice from me, but that doesn't mean my time was spent with them out of the goodness of my heart. It certainly doesn't mean I'm exploiting them by not paying them market rate. If they want market rate, they should have marketable skills relevant to the position.
The slavery comparison falls apart for obvious reasons. The volunteer comparison also falls apart because volunteers act as such to give, whereas students start a Ph.D. to get something out of it. And they do, it's just not all money. It's not even mostly money, and I think this should be clear to anyone thinking of starting a Ph.D. at this point. So for a Ph.D. student to complain that they were paid a small amount compared to someone who was employed, while eliding the vast array of other compensation they got, and simultaneously ignoring all the privileges afforded to students that aren't to employees, that to me is missing the entire value of the degree.