Stablecoins changed all that. It bypasses SWIFT all together. It allows nations to have access to dollars even if the US doesn't want them to. It doesn't add or remove and dollars from the system. But it does provide access to dollars unconditionally. That takes away the power that the dollar gives the US government over the world.
You don't understand how sanctions work.
Nothing stops officials from Iran to pack suitcases full of money and go straight to Airbus and demand 100 Eurofighters, the problem with sanctions is that nobody wants to deal with them for the fear of the backslash coming against the company. China has frequent deals with Iran, they do that not because they deal in rials or yuans, it's because they don't care, doing this through shell companies or national companies that don't care as they don't do business in the US and Europe.
Iran can embrace any coins they want, they can pay with gold or oil (as they do know), the main problem is that nobody wants to sell them the equipment they need, and they won't sell it either for usdt or for doge or bitcoin.
Besides, the whole thing has another flaw, they are going to avoid swift by dealing with a centralized coin that can be seized by us at any time and completely destroyed? I already know what's you're thinking, what if this will be a digital rial or a digital yuan protected by those governments?? But in this case, what's stopping them to use the current versions of their currency?
The US doesn't care about what stablecoins can do against sanctions, they are more interested in the problems they can generate at home, that's why the current faction in power doesn't like them.