Earlier quoted context omitted.
> on average, stock value over the long term (20+ years) is very likely to be in range of, say, 5%/year, plus or minus (actual number is not that important for our purpose here), after adjusting for inflation. Why should it be that way?
The classic explanation is that the economy is growing and thus the overall "pie" being shared is growing even if individual pieces are not as predictable. But I think broader and broader participation in the market via government policies like 401(k) has to be part of the story, and also I worry how much we try to extrapolate from modern financial history which is barely more than a single human lifetime.
The keys to economic growth he identifies are (1) property rights, (2) scientific rationalism, (3) capital markets, and (4) adequate transportation/communication. All of these appeared in sufficient form for prosperous growth several hundred years ago.
There is of course no guarantee of continued growth at same rate as last several hundred years. But given the conditions that have prevailed it has settled at a fairly stable rate as sort of a natural law.