China subsides the dollar hence the American financial Ponzi schema. They can take the carpet out from the Uncle Sam feet whenever they want, but they would damage their own export capacity. They could switch to some crypto in spite of dollar, but again they are not willing to emasculate their major customer.
In the case of Russia, they can simply switch to any other currency including their own, and if they obligate oil and gas customers to pay in rubles they could get a very strong currency︀ - but again, they would be affected in the military export, a Sukhoi or a︁ Mig are way cheaper than a Rafal or an F-35.