Consider that his position might be more profound than you considered it to be.
Mine is. It's about incentives. Now you can take it from there, and at least in my interpretation the rest of your rebuttal falls apart.
There is absolutely no equivalency to slavery. That is simply dishonest. Slaves didn't choose to be slaves. Do students who take on debt have no agency whatsoever to you? Did the people who paid such debts had no agency when paying?
If you don't like the equivalence to slavery, pick a different example, there are three I posted and more you can probably think of on your own.
We know that the idea of a rational agent in economics is a myth, and as you mentioned, it is about incentives, as well as motives.
Students who take on debt that limits them in later life don't have all the information they need at the time they make the decision. Saying the information is available is not reasonable. These students are told they _most_ go to college to make a living.
They are not told they need to get an engineering, medical, or finance degree to make going to college worth it, economically.
They are shown all the loans they can get without an equivalent amount of effort put into educating them about the consequences those loans represent. For example, how much the loans will cost in the long run, along with estimated pay for various fields of study.
Furthermore, the loans are given for any degree program without restriction.
All the comments I made about game theory still stand, and we don't need to get into the myriad problems with our education and student loan systems. I agree they aren't perfect; I just think the argument 'I didn't get my loans paid off neither should you' is an extremely selfish one. Just because someone suffers doesn't mean everyone should. Also - in my experience people who are ready to make that selfish argument are very offended when it gets flipped on them. So they can understand intuitively the issue with the selfish position.