[gorjusborg]

Also, the return is guaranteed in the case of mortgage repayment.

The returns from investment are estimated based on historical performance. You can lose money investing, not so paying off a mortgage early.

[bitshiftfaced]

At the very least, during periods when interest rates are significantly above your mortgage rate, such as now, you should put it in a money market account instead of in your mortgage. It's the same amount of risk, but it's liquid. Really, you could do a long term Treasury bond for the same reasoning (same risk, same liquidity).