What will the next move us USA you think?
USA can find all Iranian wallets and then taint Bitcoins on them. So all exchanges in USA or country that comply with USA regulations will block amounts of people that will deposit those Bitcoins to their exchange.
No fungibility = problem.
I wonder how well-versed Iranians are with mixers. In an apocalyptic scenario like that which can ensnare anyone inadvertently using cryptocurrency sourced from Iran, which isn’t even foolproof by the way - they have no way of identifying wallets of Iranian users, only Iranian services and corps and to a limited extent the government itself (it obviously won’t show US or its allies official documents that have cryptocurrency addresses on them), in such a scenario the taint chain can be broken with the use of mixers. This will also thwart your hypothetical scenario.
The US can’t ban Iran from using mixers, because most of them aren’t based in the US and don’t follow US law.
USA spends a lot of money to pay researches of chain analysis firms. IRS is doing it and also other USA offices. Chain analysis is a big business now and many companies offer it. Mixers will have hard time to stay ahead of them.