If college is free and you don't have to pay back much if you don't make much, what prevents us from wasting our resources on degrees that aren't economically useful? Sure, it is great to pay for an engineering degree and most likely that will be repaid, but if somebody wants to spend 4 years (or maybe even a PhD) on 18th Century French Art, then work at McDonalds the rest of their lives, we don't want to pay for that, do we?
When do we cut people off? Do we only pay for undergrad? When do they need to start providing value to society based on their education?
Finally, if you don't have to pay for school, wouldn't it likely encourage some people to pursue high-paying jobs they might not be really that good at? I would love to make the salary of a heart surgeon, and perhaps I could have become one, but it would have cost a lot of money and I know I never would be a great one. But if college was free, why not try?