If the chart there is right (big assumption, interest/inflation will impact it a lot!)

It's based on an analysis of historical stock market returns and already takes that stuff into account.

Basically, in order for the assumption to not hold, the market has to be so fucked that the correct investment strategy would've been ammo and freeze-dried food instead of stocks to begin with. You're really talking about weird stuff like "sovereign risks," like maybe the US becoming a fascist pariah state and destabilizing completely, or fucking with the Fed and causing Zimbabwe-style hyperinflation, or something like that. The risk is never zero, but it's incredibly unlikely (...right? 😬)

but that is about the same time my mortgage is paid off which is by far my largest expense...

Also expecting my mortgage rate to drop soon, but tempting to reduce the number of years to pay it off instead and keep paying as much as I am now.

Having a mortgage or not doesn't matter as much as you think it does, once you consider things like opportunity cost and time value of money. You could pay off your mortgage at the cost of investing less and then have smaller withdrawals from that smaller account balance in retirement, or you could invest more instead of paying extra on the mortgage and make larger withdrawals from a larger account balance in retirement. Six of one; half a dozen of the other.

Basically, it cancels out if mortgage interest rates and investment rates of return are equal, give or take risk tolerance.

If your mortgage is at today's rates (close enough to the 7% expected return of the stock market) then I guess you might as well pay it down for simplicity's sake, but if you're like me and have a 3% mortgage from a decade ago you're better off keeping it and enjoying the 4% arbitrage.