Incorrect, the DJIA lost around 90% of its index value in the Great Depression, so the theoretical portfolio in this exercise would have declined to around 10 c

Incorrect, the DJIA lost around 90% of its index value in the Great Depression, so the theoretical portfolio in this exercise would have declined to around 10 cents, but holding an asset at a negative return does not result in a margin call if one is not trading with leverage. In practice, it would have been a toss-up between individuals' outcomes because index funds as we are familiar with them today did not exist at the time. The DJIA was a price index but there was no way to invest in the DJIA basket as there is today, so brokers picked stocks on behalf of investors. So it's certain that some investors' portfolios did decline to zero during this time due to bankruptcies of all the companies they happened to be holding.