> […] you can't justify the claim that it's entirely "underlying business activity that drives total returns".

The S&P 500 index tracks earnings per share (EPS) fairly closely over the decades:

* https://www.macrotrends.net/1324/s-p-500-earnings-history

A lot of folks think the top ten stocks in the S&P 500 making up ~40% of the capitalization is bonkers, but they also make up ~40% of the net income share:

* https://en.macromicro.me/collections/34/us-stock-relative/14...

So from an earnings/income perspective, there appears to be a link between the two.

Perhaps worth noting that the US markets seem to (only?) outperform when tech is outperforming, with other US non-tech sectors basically performing the same as out countries' non-tech sectors:

* https://ofdollarsanddata.com/do-you-need-to-own-internationa...