Returns have been high in the past few decades because of rapid technological progress and growth in the skilled working-age population.
Tech progress is slowing however. Not in every field, but in most of them. And one by one, the working age population in developed countries has stopped growing and is now in reverse. The main exception are those with high levels of immigration, however those nations are either importing people with lower skill levels than the shrinking native population and cannibalizing the skilled population from other low-growth nations (eg., China to USA, Slovakia to Germany).
Another factor that led to strong returns in the recent past was the “great moderation” where returns became more predictable and with lower and lower inflation and market interest rates. This made long-lived assets like real estate more valuable. That trend isn’t done yet, put it’s in its fourth quarter.
