Well, it's true that Bitcoin would weaken the ability to implement economic sanctions. I don't think it's possible to argue otherwise.

But the argument I would make is that sanctions don't only include economic restrictions. And even with economic restrictions, asset forfeitures and penalties are only part of the sanctions that could be issued. You wouldn't necessarily need decentralized currencies to implement successful sanctions and there are other ways to gut a country's economy even if they were to circumvent such measures through Bitcoin.

For example -- if the U.S. provides military aid to a country, merely withdrawing that aid would be sufficient to compel compliance. And of course, the U.S. has military holdings all over the world which direct influence over other countries.