That’s not how that works. If you have 5% interest on your mortgage and you pay $100 more this year you save $5this year, and $5 next year.

If you put 100 into the market at 5% returns you get 5% the first year. But you get 5.25 the next year. By year 5 you are at 6% returns. By year 15 your $100 is not clearing a nearly 10% return. That $100 is just 5% flat interest until the loan is dead.

7% is the inflation adjusted average, the actual actual return with dividends reinvested is 10.26% over the last 60 years, and 7.1% inflation adjusted. A 7% flat return to compare with a mortgage rate is an actual market return of something like 2-4% in the terms of the 7%.

The reality of historical markets is you are within 2-3% of inflation at best on your mortgage rate, and stocks over a long period of time return 7% inflation adjusted. The dollar you spend in year one of a 30 year mortgage only is returning 60% value by year 30 at 2% at 7% return in year 30 you are looking at a 670% increase in value.

Ill use myself as a real world example, I bought my current house in august of 2020 at an interest rate of 2.87. If inflation dropped to literally zero tomorrow it would still be 3 more years until the interest caught up to even inflation. Obviously i am like a extreme outlier in the grand scheme of things but there is a fairly realistic chance that with the low interest rates and inflation in the world right now that my interest rate catches inflation by year 30.