> If U.S. cash is useful, so are dollars in a bank.
This is a strong assumption. So far I've experienced three cases of loosing access to my money in the bank for various reasons and I also witnessed one more case with my parents 25 years ago. In all four cases the cash was still useful, losing between 0% and 70% of the value.
None of that was in US of course, but I still think that your range of scenarios is too narrow.