Most failures under a fixed withdrawal rate regime happen during bear markets at the beginning of retirement. If such a thing happens, one might want to work part-time for a few years.
I mean, once one has a portfolio of 25 x annual expenses, he can say "hey, I can retire right now and live on 4% of the current value of that portfolio, each year. If the market plunges in the next few years, I'll get back to work, part-time, or maybe full-time if really needed, for a bunch of years. But, more probably, I'll be even richer in ten years than I am today, and then I'll know my financial needs will be fullfilled for the rest of my life, no matter what happens on the markets later".