That's fine and dandy, but look at amortization tables. The first few years of a 30 year mortgage you're only paying 20-40 dollars a month towards principal. With a 15 year mortgage it's several hundred dollars. I plan on doing the same thing and hope to be paid off in 10 years, not 25.
And I paid off the 30-year mortgage in 8 years, by putting in extra principal payments whenever I could, while not being obligated to pay more than my standard payment in lean times
That is exactly what I am doing. After refinancing my mortgage to a 15 year at 5%, I had 2 car payments plus a mortgage. When the 1st car was paid off, I added that payment amount to the 2nd car's monthly payment. The 2nd car will be paid off in a few months from now (total will be 2-1/2 years on a 48 month loan).
When THAT is done, I will put the total for BOTH cars into the mortgage until it is paid off. When I run the numbers my 15 year becomes a 6-1/2 year. So 6 years from now I will be debt-free (if the creek don't rise).
I always pay my credit cards down to zero each month so when I say debt free I mean debt free. I expect to own both my cars 20 years (one has 10K miles after 4 years and the other 4K after 3) and I pay cash for all other big ticket items by saving for them up front (I keep a separate account that I put money from every paycheck into for such purposes).
A few extra principle payments are better off being used for other purposes.