The Federal rate went to 20% during that time period - right now, they are projecting 3.4% by the end of 2023. A very different situation. (
https://www.macrotrends.net/2015/fed-funds-rate-historical-chart,
https://tradingeconomics.com/united-states/interest-rate)
Savings account/CD interest rates run lower than the fed rate. Right now the best I can get is around 1% for an online bank, with the fed rate being 1.75% after the latest hike. Figure the fed rate needs to get to that 3.4% before you will see savings accounts/CDs paying near that 2.6% rate on the mortgage. And until then, you are losing money getting the current lower interest rate - so figure that into your calc.
Could it pay off long term to hold off paying off your mortgage? Yup. But only if you have the discipline to actually
save that money once rates get to the right point, and not spend it on other things. Many people won't - that money sitting in a savings account would suddenly be the reason to take a vacation they otherwise wouldn't, or buy a new car, or do a renovation, or just go out to eat a whole lot more. In that case, it's better to just pay off the mortgage.
And note the idea that it could "pay off long term" is a bit tenuous, since any money in savings could still be losing you money due to inflation. You could end up with more money in savings by not paying off your mortgage, but it will still be worth less than what it was the day you earned it. The only way to hedge against that is to go with different investments, and you have to be able to tolerate bear markets like we have today without freaking out...