It doesn't track. I didn't buy out of my means when I purchased a home at a low rate. In fact, it allowed me to afford a better home for a cheaper price. You are not suddenly worse off at lower interest rates, infact, you owe the bank less. That doesn't mean the duration of your loan changes. Referencing your example, you can still get the same term loan (12 yr is what you used, but in the US that would be an unusual duration), and the loan itself is cheaper.
Why would I give the bank extra money?