Russia replaced the dollar mainly with Gold and Chinese Renminbis, not with Russian Rubles.
Their economy is now more resistant to sanctions than it was in 2014, and the slowed growth was actually in part done for that exact reason (the article I posted above explains how).
China won't need to donate any money to Russia, just sell them what they need and buy their gas, oil and agricultural products (even at a discount) will be enough for Russia to survive, especially considering the oil/gas prices will be higher anyway because of the sanctions.
Of course the sanctions will hurt, and there is still no way to predict for sure what effect they will have, but talking about Russia crumbling with certainty in the face of sanctions is not quite clear to me at the moment at least.