The two largest economies in the world are the US and China. Between them is the largest trading relationship in the world. How can China sell goods to the US without accepting US dollars?
Setting aside the US impact, consider if China stopped all trade with the US. How could China's workers stay employed in that environment? Unemployment would skyrocket in China - and discontent. I can't speak for China's leaders, but based on what they target with censorship, I think stopping discontent is their top priority. I think avoiding discontent is more important to them than money in USD that might get frozen.
As to Russia's central bank, can it conduct it's mission without USD and Euros? I suspect those are the most traded currencies. In normal times they might defend their currency from small drops - selling USD and buying rubles, to influence currency markets. But if they lose all influence over the largest currency markets (USD, Euro?), that limits their options and might spoil their ability to act.