Earlier quoted context omitted.
that's effectively only ~4 observations then (100/24). i wouldn't want to make a very important life decision based on data from only 4 observations. with only 4 observations, "common sense" and other forms of first-principles reasoning are much more important than the 4 data points. also one should consider data from other countries and other historical eras.
It's not though. There are ~75 24-year windows you can use between 1913 and today. I suspect you will find similar results with almost all of those windows. Obviously past performance is not an indicator of the future, but 75 data points is a pretty decent amount of data.
My point is that having 100 years of investment data is much more like having 4 data points than it is like having 75 or 75 *12 (if you cut the years into months). Even though you literally have 75 data points, they are pretty close to a copy-paste of each other; not statistically independent.