Lenders are incentivized to loan as much out as possible because their risk is so low, and so there is no reason to NOT approve a $150,000 loan to a student earning a degree which will realistically net them $40,000 a year.
As soon as you let borrowers default, the math starts to make sense again. Lenders will have to evaluate how much they are willing to lend a student based on their expected earnings post-graduation. Universities won't be able to increase tuition at such an inflated rate since there wouldn't be infinite money being pumped into the system. College may actually become affordable again over time.
It's not a mystery what the actual problem is here.
edit: corrected "lenders" to "borrowers"