[tsimionescu]

Yes, they could declare any exchange that offers XMR to customers as banned, and no financial institution that trades with that exchange would be allowed to trade with a US bank. That's how US sanctions typically work. Most likely, that exchange would quickly stop offering XMR trades itself.

And even if they didn't, that BTC could indeed no longer be allowed in the US controlled banking system (which is far larger than the West).

Of course, if your friend were able to redeem buy actual goods for XMR and send those back to you, great. But what goods vendor would want to own XMR in a world where no institution that does business with banks would want it?

Note that what I'm saying is not purely theoretical - this is the status of Tornado Cash - where they went so far as to arrest developers of that. And banned BTC also exist - there are BTC that are illegal to own, and wallet addresses that are illegal to trade with.

[notch898a]

Your points are taken, may rebuttal would be that monero is only worth at most about $3B and that much value is easily captured outside the banking system. It seems like one of those scenarios that will require a gargantuan effort in practice and at best you end up with drug-war style penalties without even choking out the lions share of the value, and even if you do something else just takes its place. It would really pain me to see my country going down that path again, but history does have a way of repeating itself.