Presuming you are saving for later. Besides a margin call, you might want to exit for, buying a house, going into retirement, covering a period between jobs, or to deal with a medical emergency.

Doing that during a crisis hurts. This risk diminishes the value of investments as a safety cushion.

Index funds aren’t a safety cushion. Vanguard rates it’s own S&P 500 index fund as a 4 out of 5 for risk. [0]

[0] https://advisors.vanguard.com/iippdf/pdfs/FS540.pdf

> Besides a margin call, you might want to exit for, buying a house, going into retirement, covering a period between jobs, or to deal with a medical emergency.

In all of those scenarios you should not be in stocks/equities in the first place. If there is a possibility of needing cash with-in the next 5 years, that money should be in either bonds or term deposits.

One's downpayment, first/next few retirement years' income, and emergency fund(s) should not be in equities.

After 1989 it took the Nikkei more than a decade to finds its bottom (and then another bottom in 2009) and it hasn't recovered since. How many years of "discount buying" are you planning in?

Systemic risk. If a ton of smaller companies see their shares plummet all at the same time, there might be repercussions for the economy as a whole. It’s true that individual investors might be able to ride it out (and the same was true in 2008).