> A small number of companies are almost 20% of market value.

That's been a 'tradition' for a long time:

* https://awealthofcommonsense.com/2020/07/the-nifty-fifty-and...

* https://theirrelevantinvestor.com/2018/11/26/the-nifty-fifty...

* https://en.wikipedia.org/wiki/Nifty_Fifty

* https://etfdb.com/history-of-the-s-and-p-500/

* https://www.qad.com/blog/2019/10/sp-500-companies-over-time

It's just that which companies have been the top 20% has changed over the decades.

The profits are far more concentrated than before, and there are fewer public companies than before.

https://www.nytimes.com/2018/08/04/business/shrinking-stock-...

You can also easily see this in the reduced bargaining power of the bottom 3 to 4 quintiles of the American people, as evidenced by their stagnating or declining pay and higher volatility in the demand for their labor.

https://www.pewsocialtrends.org/2020/01/09/trends-in-income-...

Are you familiar with the "past performance" disclaimer?