Dynamic Retirement Portfolio Sustainable Withdrawal Rate

How can retirees estimate whether their investment strategy will sustain all the withdrawals they expect to make in retirement? In his February 2021 paper entitled "The Sustainability of (Global) Withdrawal Strategies", Javier Estrada  presents two tools to monitor retirement plans for early signs of trouble:

1. One evaluates sustainability of an existing withdrawal strategy.
2. The other calculates the sustainable level of inflation-adjusted withdrawals at any given point during retirement.

Both change over time according to (1) deviations of actual returns from those used in modeling the retirement portfolio and (2) the number of years left in retirement. He also illustrates these tools for a sample of 21 countries and the world over a 120-year period assuming a 30-year retirement with 30 annual withdrawals at the beginning of each year. The terminal value one year after the final withdrawal is the bequest. All returns and portfolio values are in real (inflation-adjusted) terms. Using annual real total returns for stocks and government bonds for 21 countries in local currencies/inflation rates and the world in U.S. dollars/inflation rate from the Dimson-Marsh-Staunton database during 1900 through 2019, he finds that: