A new Apollo chartbook (https://www.apolloacademy.com/wp-content/uploads/2026/02/020726_Public-VS-Private_Chart-Book_v3.pdf) on the declining importance of U.S. equity markets in the economy shows that S&P 500 companies are only ~18% of U.S. employment.

But context and history matter. Here, as i've reconstructed it, is the extended series. The share largely flatlined over the period. I'm not sure that's "declining" importance, but it is interesting.