"Will those later [student loan] debts be forgiven, too? If not, the plan would create a generation of student loan lottery winners..."

"... with losers on either side. People who had already paid back their loans would get nothing. People with future loans would get nothing. People with debt on the day the legislation was enacted would be rewarded. If, on the other hand, the legislation creates an implicit promise that all kinds of future student debt will also be forgiven, it could have unintended consequences. The Sanders and Warren plans control the cost of public undergraduate education by setting tuition to zero and keeping it there. So financing public higher education would become a matter of the federal government and states deciding how much they want to spend on higher learning. The universities would have no pricing power, because there would be no prices."

From "Canceling Student Loan Debt Doesn’t Make Problems Disappear/Plans from Bernie Sanders and Elizabeth Warren could have unintended consequences" (NYT).