This is a form of leverage, but how do you use it in a low-risk way? Fixed income investments seem to be near zero, and maybe people don’t want to invest more in stock.

If the alternative is cash sitting in a bank account, paying off the loan looks better.

Edit: I looked at Series I, and it's variable rate with a 3-month interest penalty for early withdrawal, so you're not going to get 7% in the end. It seems worth doing a more detailed calculation.