Pretty sure most people just sit in the default requirement 20XX year funds, which heavily weight away from equities once people are retirement age.

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.

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.