There’s surely an important distinction between FX risk (particularly associated with bonds denominated in a loose/weakening currency) and default risk.
FX risk is always substantial, and doesn’t require any kind of triggering “event”. Check out the long term USD charts against GBP, JPY, CHF etc. Since the mid-20th century foreign holders of USTs have lost enormous amounts of the principal value of their bonds. Does that stop people buying USTs?
Reserve currency me arse!