Ok, but what about the point in the article about the underlying not being traded in 96% of the index trades? Isn’t this hiding valuation, liquidity, or some other aspect of the underlyings?
Is it going to consequentially drive valuations higher than they've ever been compared to history? Sure. But isn't that more of a paradigm shift than a "bubble that may one day pop"?
With more and more people having easier access to investing in index funds... why would it ever go backwards? You'd need people to be sick and tired of risking their money and not having anything to show for it (aka no/poor returns).
All stock prices move based on supply and demand not theories and strategies.
Well, why do people demand a given stock? One word: dividends. This may seem obvious (or not!) But Robert Schiller in a MOOC some years back made this basic truth clear to me: if a company would hypothetically never pay a dividend, no investor would ever want it. Stocks do not need to trade in large volumes to bring people profit and be desirable. They just need to bring dividends, either now or in the future.
There are other ways to make money from stocks - and stock appreciation is likely a bigger driver to the average investor (and most pro investors).