If the recipients of this largesse go on to earn higher incomes as a result of their education the nation will then receive more in income tax over their lifetimes than it otherwise would have. It could be considered an investment rather than a giveaway.
According to the linked article, that's not how Sanders is playing it. His proposal is to pay for the program not out of the income tax dividends from the program, but from new taxes which will be placed on financial transactions. You can see the context he provides in the unveiling of the proposal. This is as much about sticking it to Wall Street as it is about fixing college tuition debt. Probably more so.
Raise taxes on Wall Street. Fight the pushback and attract young voters by saying it's to make college free. Everybody believes in free college! Only a monster would question taxing Wall Street to make college free! Etc.
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Besides, if it's an investment for the state to pay for college degrees, then it's an investment for the student as well, and suddenly their student loan debt isn't a problem the state needs to solve after all. The students will solve it on their own, by getting their degree, paying off the loan, pocketing the profits, and enjoying their improved quality of life. The state still gets the same income tax windfall, even - and they don't have to raise a single cent of new taxes to do it!
The problem is that a college degree isn't always a good investment, especially at current prices. If the idea is to realize a tax windfall from secondary education, then the smart approach would be to first try to reduce costs, and second try to take on those costs only for degree programs which actually have a high chance of profitability. A program that subsidizes qualifying (meaning demonstrated cost-effective) STEM degrees, for example, would make the kind of sense you propose.
Just blindly paying for everybody's college, and raising new taxes on some other sector of the economy entirely to pay for it, makes no sense at all for your proposal.