If private loan repayment rates were capped, then private institutions would have a very, very strong incentive not to loan student loan rates larger than a certain amount to students, depending on the major and university. Quite frankly, a student asking for 200k for an english degree at Drexel would be denied. Why? Because Drexel isn't known for its english program. Average salary of a Drexel graduating English major is, let's guestimate, 50k a year for the majority of their lifetime. At 50k a year, at a 10% rate, the bank isn't going to get its funds back for 40 years, before interest. If the average age of death/retirement in this country is in the early 70s, the student will literally die of old age or retire before finishing repaying the loan.

Now students will be unable to attend financial unviable options. The university will understand that if it raises its tuition beyond a certain level, it will be unable to support its english department, because no sane bank would back the loans required to graduate.

Again, the above is a very contrived example with BS numbers. And I'm not suggesting that the above is the way we would want to fix the issue. But it could work, and it certainly puts pressure on the universities.