I agree. Keeping it to 40 companies does not seem realistic today with all the diversity in our economy. Back in the 1920s, nobody could imagine that gigantic corporations would emerge from such things as streaming video (Netflix), fast food (Chik-Fil-A), home delivery (Amazon), and car rides (Uber), just to name a few.
Given how many changes out of companies the DJIA has made (and all of their mathematical machinations accompanying those changes), there would seem to be no reason to not change the number of companies tracked.
It would be interesting to track the fraction of the stock market value represented by the DJIA vs. the total US stock market value over time.
The S&P 500 index is a more accurate depiction of the overall market.
https://us.spindices.com/indices/equity/sp-500
The S&P 500® is widely regarded as the best single gauge of large-cap U.S. equities. There is over USD 9.9 trillion indexed or benchmarked to the index, with indexed assets comprising approximately USD 3.4 trillion of this total. The index includes 500 leading companies and captures approximately 80% coverage of available market capitalization.
Here’s the reduction of the argument/question:
If you don’t keep the same companies over time, why keep the number 40?