FWIW, I am splitting the difference: I am keeping my very low mortgage while I am still employed, because my money is much better invested in the market; but I have also timed the mortgage to be paid off by my planned FIRE date, so I have the security of lower mandatory spend once I no longer have a steady income stream from an outside source.
I know there are many who take the position that you should take your mortgage to the grave with you, because keeping that extra money invested in the market will give you better returns regardless of whether you are 35 or 75. But achieving and maintaining FI requires two conflicting considerations: maximizing your gains; and minimizing your downside risk. IMO, when you are in your asset-building years, it is appropriate to put more emphasis on maximizing gains, because you have the safety net of a consistent paycheck to carry you through any market downturns. OTOH, when you are FIREd, it is appropriate to shift your emphasis to limiting risk -- not entirely, because you still need your assets to grow, but since your safety net of current income has gone away, it is reasonable to also get rid of leverage so you don't have to worry about pulling enough out of your investments to cover those debt payments.