If people actually dumped index funds for cash en masse it would be catastrophic. To attach some numbers, MSFT averages about 35M shares in daily volume, and that includes all the market makers, HFTs, etc. BlackRock (iShares) owns 593M shares of MSFT and Vanguard owns another 482M. Together, the amount of shares that index funds own is about a month and a half of total trading volumes. I'd bet that such a crash would unfold over about 2-3 days, which brings up the specter of stocks literally going "no bid", where there are not enough buyers for every seller to sell, at any price.
Likely the government would step in and inject cash directly into the markets to support them in such a scenario, because a broad-index stock market crash is the modern-day bank run. Retirees carry the bulk of their savings in the form of stocks; if it disappears, we'd likely face revolt.
If you hit sell on a vanguard ETF and it sells on the market, then Vanguard isn’t the buyer is it? So in that situation with everyone dumping ETFs there would be a lag on the time taken for the ETF to sell and Vanguard to then dump the stocks back out in the market. It’s never occurred to me the situation where huge numbers of people dump index funds and how Vanguard/Blackrock account for that without becoming bag holders of the underlying stocks themselves.
In any case, I’m not sure that large enough numbers of ETF holders are sitting close enough “to the button” to hit sell in the event of a sharp downturn occurring over the space of even a week or two. And a lot of them would see it as an opportunity to DCA into the dip anyway.
If it's an ETF it's a little complicated. The usual mechanism for selling an ETF is that there's a buyer on the other end who's buying shares in the ETF itself, not the constituent stocks. Arbitrage keeps the price in line with the index constituents; if the ETF diverges from its constituent assets, some HFT can buy the ETF and sell the constituents and that will force them to converge.
However, most ETFs are also setup such that they can create or destroy shares in response to large shifts in demand. In this case, if enough people hit sell, the ETF itself will buy back shares and use the proceeds to sell the underlying assets, in a transaction that mechanically should be market-neutral and just propagate the supply/demand of the fund down to the individual stocks.
With Vanguard specifically, it's even more complicated, because VTI is not a separate ETF. It's a share class of the Vanguard Total Stock Market Index Fund. But the mechanism is largely the same - it has the same Authorized Participant system to mint new shares in case of high demand and redeem shares if everybody sells, and then passes these requests on to the underlying mutual fund, which can then piggyback on some of the tax efficiency benefits of the ETF.
Same old story of too big to fail. The government will "inject cash", that is borrowed, so that retirees 401k accounts don't go down. But who pays back the borrowed funds? The non-retirees. Everything is optimized for the boomer generation to be fine, who cares about anyone else?
If you're retired and that exposed to stocks then you deserve to lose the money you risked.
Pretty sure most people just sit in the default requirement 20XX year funds, which heavily weight away from equities once people are retirement age.
Market makers aren't included in those numbers, Vanguard, etc don't trade normally but on secondary markets most of the time.