As you hinted at, and as Paul Samuelson famously said, we only have 200 years of history to go on, and the experience of the rest of the world, as well as current expected returns suggest, going by those 200 years are overly optimistic.
Forget all the sophisticated methodologies: GIGO, and what goes into these black boxes is most assuredly G.
Here are 2 simple ways of looking at it:
- Start with 3.5% real for stocks, and 2.5% for bonds. that's about 3% for a mixed portfolio. if you're going to retire at 50, your time horizon is for all practical purposes "forever," so you can only withdraw your real return, or about 3%. but it's worse than that, since you have to adjust for uncertainty and a bad initial draw. so figure 2%.
- Even simpler: since in the long term, to stay hedonically adjusted you don't just have to keep up merely with inflation, but with the living standard of your nonretired peers, which increases at the productivity growth rate, or 2%. add in a soupcon of uncertainty and your hedonically adjusted rate of return is zero. so . . .you have to save one year's living expenses for every year you plan to live, or 50 years, "worst case," or . . .2%
Myself, I plan to live what passes for a "frugal" standard of living in today's society (I'm fortunate to have been raised in the 50s, and to have sense of appreciation of what I have now) and spend only 2%, maybe a tad more as i age, with the knowledge that I'll most likely be leaving my kids, grandkids, and charities a nice chunk of change. which provides its own rewards.
Note that a 100% safe rate is pretty much redundant, since as history shows, you are likely to get caught up in historical events that makes worrying about your financial assets the last thing on your mind. We don't live in a super safe world, if history is any guide, so all the retirement studies have an precision that don't live up to its accuracy. John Greaney is well aware of this, and lives on 1% of his assets. It would be wise to do as he does.