In practice, it will be the day that Vanguard stops as an ongoing concern and returns the funds to its customers.
The popular way of raising monies via equity (instead of bonds) on a mass scale is less than a century old and the American way of equity for retirement only a few decades old(*) and not by any means universal... so it would not surprise me if this way of doing things will be replaced on a similar timescale.
(*) And the 100% equity notion only a few years, thus perhaps hinting at something like a bubble if it weren't for the fact that the bond markets are still far larger than the stock markets... but at least retail investors are now trying as hard as possible to change this.
OTOH, global trade is literally ancient as are financial markets. But not as you and I know them.
What I expect is that various sectors of the economy will not fold simultaneously but rather one after another. As an example, consider the US auto-industry. Financially, it's been decimated, but it still exists and more importantly, you could have sold your position and moved the money to some other sector leaving some other person with the bag. Economically, US car manufacturers are considering killing of the sedan (to focus on SUVs). This means that sedans now must be supplied from Japan and South Korea.
https://www.bloomberg.com/news/features ... -for-death
Or just consider the US manufacturing sector.
So analogously to the supply of electronics, the capacity to construct sedans is going away locally (that's economic destruction), but since the financial markets are here, it's easier for an INDIVIDUAL to avoid the financial destruction. If you still want a sedan (for your taxi company), you now need a trans pacific supply line. Conversely, if you just need to go from A to B, you can pay for it with your finances and get whatever transportation your local area still provides.
TL;DR - I don't think the financial sector will be the first to go. The ability to transact without trust (in the counterpart) is simply too valuable. I do think that all the fancy structured products and high-speed trading will be dialed down simply because the energy costs and technology competition demands are too high. That also goes for crypto. (I can hear the nerds scream now )