People will respond to the incentives that are given to them, regardless of ethics.

The question is, what system would avoid this perverse incentive? A graduate tax would have the same problem. Free tuition shifts the burden to every taxpayer unfairly. Maybe another repayment scheme, but that would disproportionately harm poorer students.

To tell you the truth, I'm not sure what tools a creditor has to pursue a debtor who has left the country in general...