China is hemmed in and is more interested in making money.
Russia is no longer able to project power.
Maybe. China is not as inward looking as you think. They are trying to put a base in the Azores, and have operations in Africa.
Russia dearly wants to reclaim its empire. If the US stepped back, they might decide to make a go of it again. How successful they will be is an open question, but it would be foolish to write them off.