Wouldn't calculating CAGR this way be misleading because it assumes you will keep the loan for the entire length and won't refi out.

Maybe. There could be a scenario where you do the math as I suggested and refi, then interest rates fall further but the math is no longer favorable. You would have been better off if you waited.

Or, there could be a scenario where you do the math as I suggested and refi, then interest rates fall further but the math and the math is favorable again. You refi twice, but not as efficient as waiting and refinancing one. Again, you would have been better off if you waited.

However, without having a crystal ball on future rates, it's is a good an accurate way to compare two scenarios: keep current mortgage for life of loan, or refinance.

This is especially important when rates are expected to keep falling well into 2021, i.e. you do want to keep that option to refi-out in hand.

Isn't this the same thing as saying the bond market is inefficient and easily exploited? I see no reason to think I or anyone else can reliably predict interest rate trends.