As eager as I was to pay off my mortgage back in 1997-98 and put ER onto my radar for the first time, I was ambivalent about how the math would play out.
The markets were beginning to explode in their late 1990s boom, so the question about where I was going to get the money from mattered. But I was also happy to take some of its early gains off the table so I could guarantee them.
Then, there was the interest rate on my one-year ARM which was creeping upward since I refinanced my mortgage in 1992, from nearly 11% (I got the mortgage in 1989 when interest rates were still sky-high) down to 6%. By 1997, it had crept back up to around 8%.
But what actually pushed me over the edge was running the numbers some more and discovering that I would no longer need to deduct mortgage interest on my STATE income tax return, instead being able to take the standard deduction. Therefore, there was no state income tax increase by having less interest to deduct on that return.
Even then, I was still hedging my bet by paying down the mortgage in chunks. The first was in April of 1997, then see what would happen after that as far as market gains to replace the money I used to pay some of it off. I used a mix of a bond fund and a mixed-asset fund to pay the first chunk. The second chunk, in early 1998, I used only the mixed-asset class fund. When I was informed that the interest rate was going to rise some more (to just over 8%) on its anniversary date (May), I decided to pay off the rest of it, which by then wasn't a lot.
I can't say I would have paid it off had my interest rate been only 3%, especially with a similarly booming market. Maybe I pay some of it off early.
The markets were booming so much in 1997-98 that the temporary reduction in portfolio value from those chunks were very quickly eliminated by market gains.