But the bigger question is would they be able to fully detach themselves from USD? Will all their trading partners be amenable to avoid using the USD? If not, could they survive trading only with those that are willing to accept their local currency? Are international trading partners willing to deal with an unstable Iranian rial, for example, that has been losing value so fast in the past several years?
To my mind, the gravity of the impact of ditching the USD wouldn't be as heavy on the USD as their very own countries.
Both countries Russia and Iran, can only accomplish ditching the USD if they decide to trade between themselves alone, but we all know that is not a good economic plan. They still need trade relations with other countries that might not be willing to do international trade with another currency other than the USD. In that case if other countries are not willing to accept another currency for international trade, wouldn't that be a flop to them? They need to first rally other OPEC countries and industrial countries like china, to be able to get a head start, or they remain in trading between themselves which will not be favorable to either of them in the long run. US has a lot of influence in many countries, therefore it'll not be easy to replace USD as the dominant force in world currency domination.