Well argued Ben. However, there are two additional and critical pieces of the puzzle that make it even more beneficial to invest rather than prepay your mortgage principal, as I explain here.

First, when you owe money, inflation is your friend, eating away at the real value of your loan balance, and in many cases even faster than your monthly payments do.

Second, and even more important, if you use extra cash to prepay your mortgage principal, but then lose your job with even just a few years to go before your mortgage is all paid off, those extra payments won’t save you from foreclosure. In contrast, had you invested the extra cash, you could have used that investment to keep making your monthly mortgage payments until you find another job, saving your home.