If the USD and hence fist currency crashed, what would happen with our assets?

Typo in title: fist - > fiat

Short question is: if my wealth is mostly into index mutual funds, globally diversified, what would happen when/if the fiat currency system crashed?

In theory, our assets are just that, assets. But we track them in currency value. As I understand, we wouldn't lose their value (not accounting for massive market fluctuations that would happen in such event).

However, we wouldn't be able to "sell" either, right? Because our brokers and banks won't give us milk, rice, or bitcoin (any other tradeable currency) for our assets. They will only trade them for fist currency. Effectively leaving our assets locked, so effectively being as if they became 0 value as well.

Maybe I'm over thinking, but I'd like to know people's thoughts on this.