But yet there AMZN, GOOG, and others sit, responsible for a large part of the return in Bogle's revered index funds.
If you knew with 100% there will never be a buyback or dividend by AMZN while you are a holder of their stock, arguably you are making the same trade as someone buying bitcoin or gold: expecting someone else to come along (who perhaps expects to have a longer lifespan) to pay you more for the stock than you did.
Note: I am not arguing that buying AMZN is a bad idea, I am saying that the argument that there is no "underlying rate of return" for bitcoin is a poor argument coming from someone who is also telling investors to buy those stocks, many of whom will certainly never see direct yield on their investment, just capital gains.
I think the difference here is that people can intuitively justify changes in the value of a stock regardless of buybacks and dividends because company performance is a tangible, measurable factor (even if only by proxy of flawed but useful metrics). The value of gold is more vaguely determined by market perception/speculation (e.g. gold doesn't have quarterly earnings reports with detailed accounting). Ultimately something has value as long as people believe it has value, but I think the discomfort from folks like Bogle comes from the lack of a rigorous method for valuation vs. assets like equities.