Bonds are basically like high-denomination dollar bills with a small "coupon" interest payment stapled to them. The interest can be ignored and then we just treat them like money. As all big financial institutions do. They're the "risk free" place to store money. Which is why Circle (via Blackrock) are holding so many in the first place.
Now, what happens if the US defaults, even technically-defaults (missing just one payment) and all this "money" held by financial institutions is suddenly "not money"? Or "maybe not money"?
I would expect a majority of US banks to be in FDIC resolution the next day. As bank stocks fall off a cliff, they start taking the rest of the economy with them. Things would rapidly outstrip the existing ability of the system to absorb small crises. It would look like Cyprus in 2008.
