Isn't this just "shockingly simple math" in reverse? You plug in years and it gives you a savings rate, instead of the other way around.
Sort of, but the main result of this research is to reveal information about long-term market effects, and I see it only peripherally connected to savings rates and withdrawal rates.
The main conclusion is that over a long enough period, there is a "reversion to mean" on investment returns. It's saying that someone who has relatively shitty returns over their 30 year working career is still likely to be ok in their 30 year retirement (even though their withdrawal rate might be much higher than 4%), because those second 30 years are likely to produce better-than-normal returns.
If an addendum was made to "The Shockingly Simple Math", it would say something like this: "After you've saved X% for the Y years prescribed by the Shockingly Simple Math, your stash may be much smaller or much larger than you initially expected it to be, due to an investment environment that was different from the assumed average. But don't worry about that final number, or working another year. Just retire and withdraw the dollar amount you originally expected to, and it'll all work out!
If you retire with a smaller-than-expected stash, your withdrawal rate
will be greater than 4%, but that's fine because you're gonna get kickass investment returns throughout retirement. If you retire with a bigger-than-expected stash, your withdrawal rate will be less than 4%, but don't get greedy and increase it to 4%, because you're going to get some shitty returns going forward."
Of course it seems a bit risky to wholly rely on that "reversion to mean" (especially in the case of early retirement where the working period covers a rather short amount of time), but it's interesting to see the data show that it basically would have worked in the past for normal retirement timelines.
It's why I don't actually feel much more "FI" now than I did a year ago, even though the numerical size of my stash is some 30% larger. A "reversion to mean" could quickly return my stash to the size it was a year ago, and if it does, I similarly won't feel much
less "FI" (I hope!)
It also helps explain that people who have such specific FI numbers/dates aren't quite as misled as I thought they were. It used to baffle me how someone can declare FI in, say, June of 2013, given how much market volatility sloshes a stash around (especially when it's near FI-size). But this study essentially says that the sloshing can be ignored, and as long as you did the prescribed savings it'll all work out.
