I suppose that (barring any fundamental transformation of the structure of our society during the intervening time) the income tax rates will still be progressi

I suppose that (barring any fundamental transformation of the structure of our society during the intervening time) the income tax rates will still be progressive, and that the lowest brackets will be quite low - perhaps similar to the current ones. If you assume that, and you've paid off your mortgage by the time you retire, and aren't planning to blow lots of money on air travel and hotels, then probably the rate at which you withdraw from your 401k will place you in one of those low-rate, bottom brackets.