The data gains strength when compared to the history of major concentration bubbles: Nifty Fifty stopped at 40% in the 1970s, Japan at 44% in 1989, and the TMT bubble at 41% in 1999.

In other words, the current concentration is stuck at the historical ceiling that preceded significant corrections.

The only episode that went further was that of the railroads in the 19th century, which reached 63%.