It's too simple because it's a point in time snapshot. From your link: If you save a reasonable percentage of your take-home pay, like 50%, and live on the re

It's too simple because it's a point in time snapshot. From your link: If you save a reasonable percentage of your take-home pay, like 50%, and live on the remaining 50%, you’ll be Ready to Rock (aka “financially independent”) in a reasonable number of years – about 16 according to this chart and a more detailed spreadsheet Example numbers: Take home: 50k Spending: 25k Ok sure we can say that if nothing changes and historical returns approximate themselves then this person can retire in 16 years. But that's not going to happen. Things are going to change. Say on year 5 after career advancement + kids it changes to this: Take home: 100k Spending: 75k Now the numbers point to a different retirement date (even though the annual saving amount is the same). Which means that the original 16 year estimate is already out the window. Then on year 6: Take home: 40k (lost job, looking for a new one) Spending: 60k (still got to pay the bills - from savings) Now the numbers from year 1 and year 5 are even more further off. And the simple snapshot says they'll never retire now due to a negative savings rate.