The fundamental purpose of the so-called “4% rule” is to determine an appropriate “income” or spending floor that is low enough to survive potential sequence-of-return risk – at least, based on the worst return sequences that can be found in the historical data. If market returns going forward turn out to be as bad as scenarios like the Great Depression or the stagflationary 1970s - from which the 4% rule originated - the portfolio is still anticipated to sustain inflation-adjusted spending to the end of the 30-year time horizon. If returns are better, there will simply be “extra” money left over.
Yet the reality is that in the overwhelming majority of scenarios, returns are not so bad as to necessitate a 4% initial withdrawal rate in the first place. In fact, by applying the 4% rule, over 2/3rds of the time the retiree finishes with more than double their wealth at the beginning of retirement, on top of a lifetime of (4% rule) spending! Half the time, wealth is nearly tripled by the end retirement, as retirees fail to spend their upside!
Accordingly, a more dominant approach is to plan in advance that if the portfolio gets “far enough” ahead, spending can be increased - but not increased so quickly that the retiree might have to go backwards shortly thereafter. Of course, the reality is that retirees experiencing a “favorable” sequence of returns will inevitably sit down for a portfolio/retirement review and realize they are so far ahead that it’s “safe” to increase spending anyway. But as it turns out, a relatively simple ratchet-style approach, where spending is increased by 10% any time the portfolio rises more than 50% above its starting value, can “dominate” the traditional 4% rule, generating equivalent or better retirement spending in all scenarios, even while being conservative enough to not require a spending cut in the event of a market pullback in the future.