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If you pay off the mortgage, you will have a bunch of equity that is rather ill-liquid and inconvenient to access should you need the money for something (especially if you need money on short notice).
If you did want to take out a mortgage again at some future date, however, as long as you have documentable retirement income of some kind (even if it's only a little) and you keep the loan-to-value below 60%, pretty much any mortgage lender out there will give you a loan on decent terms unless for some reason your credit score is low.
I do understand your concern about your 401k. Maybe you should shift the kind of investments in your 401k. I don't think the old "buy stocks and hold 'em long" is gonna be a successful strategy anymore for at least the next 10 years or so.