Politically unsustainable?

Prime Minister John Key is giving up too quickly on student loan interest rates.

Charging interest would bring in considerable extra revenue for the Government, but Key said he would be voted out if National did so.

"Bluntly, if you want me to be really crude about it there are 565,000 student loans out there. If we add interest back on the student loans, it doubles repayment time of the loan.

"If your loan is $50,000, and it's estimated it will take you eight years to pay it off, we effectively turn it into a loan that is about $90,000 with interest that takes you about 15 years to repay," Key said.

"That is about the only thing that will get [young people] out of bed before 7 o'clock at night to vote, but it's not politically sustainable to put interest back on student loans. It may not be great economics, but it's great politics. It is a bit of a tragedy because it sends the wrong message to young people, it tells them to go out and borrow debt." [emphasis added]

What would be so hard about instead saying something like:

"Starting next year, new student loan borrowing will be subject to interest. Existing loans will stay at zero percent. We do not wish to impose an unexpected burden on existing loan holders. But current policy encourages ridiculous borrowing behaviour."

Would students really get out to vote en masse if new borrowing were subject to interest charges? Would Labour really look at the current deficit projections and reckon promising a return to zero-percent loans a great idea?