I like that concept of building up your investments until you could cover the mortgage in one fell swoop, and then decide whether to actually pay it off or not.
IF, repeat IF, you are going to pay off the mortgage early, this is the safest way to do it.
That does not make it the safest choice, just the safest when it comes to paying off a mortgage early.
Any personal opinions on paying off in a lump sum at the time of FI based on relative size of mortgage versus stash?
I'm only speaking to the US market. The facts are different in other countries which means different results, thus different choices.
Historical average inflation is about 3% in the US.
That means in year one I have to earn $1 after tax dollar per dollar I pay on my mortgage.
With average inflation, in the 2nd year of the mortgage, $1 is only worth $0.97 in today's dollars.
What does that mean? If your income adjusts upward to cover inflation, you just got a 3% discount that year.
It gets better.
In the 3rd year, again assuming average inflation, $1 is only worth $0.94 in today's dollars. That's a 6% discount.
If your income grows as it adjusts to inflation, your mortgage consumes a smaller and smaller portion of that income.
By the time you get to year 30, that mortgage is trivial.
Now, if your income won't adjust to inflation because you chose an annuity that doesn't have inflation protection, you're screwed.
Now, if your mortage rate is 18% -- yes, it's been that high in living memory -- that SOB of a mortgage needs to get paid down pronto. Pour money into it until you can get it refinanced. (Or better yet, never get one like that!)
But if your mortgage rate is 3%, the world is your oyster.
The other thing I can think that plays into it is paying it off would mean a lower manufactured income in RE and therefore lower taxes, versus pulling enough out to cover a mortgage payment.
If I'm paying for my mortgage by withdrawing funds from a 401K or IRA, my income just went up by the P&I of my mortgage.
If I'm paying for my mortgage out of taxable accounts, my income only went up by the amount of "profit" the stocks I sold have had. Plus, unless my income is very high, that's taxed at a very low rate - or even not taxed at all.