There is no reason to move to corporate debt to back deposits. There is no US credit risk because the US can always print more dollars. Because the debt of other US companies is also denominated in US dollars, it is equally exposed to inflation as US government debt.
US treasuries becoming undesirable doesn't really have an effect on stablecoins, because (at least for USDC and I believe USDT) they hold short term treasuries that expire in a few months. The operator of the stablecoin can just hold the treasuries until they expire. Also if the operator of USDC can't pay out all claims right now, they will be in a few months so an oportunist market maker can buy USDC for 98 cents on the dollar and cash out 100 cents a few months later.
A high inflation rate can significantly reduce demand for US government debt. Same applies to corporate debt - that's why inflation indexed bonds exist (called TIPS for US debt). They are not a big part of the bond issuance atm but they could grow massively. In my outlined scenario the US gov won't be able to issue more TIPS because of their fiscal situation while profitable corporate issuers could.
I'd say that the risk of the US defaulting on Treasuries is definitely higher than zero... but if that happens then we'd probably have bigger fish to fry than stablecoins.
US treasuries becoming undesirable doesn't really have an effect on stablecoins, because (at least for USDC and I believe USDT) they hold short term treasuries that expire in a few months. The operator of the stablecoin can just hold the treasuries until they expire. Also if the operator of USDC can't pay out all claims right now, they will be in a few months so an oportunist market maker can buy USDC for 98 cents on the dollar and cash out 100 cents a few months later.