I think you make a lot of good points except this one

They would collapse under their own weight in a year if cut off from the rest of the world.

What you really mean is cut off from the US Dollar. They would still have Russian oil and fertilizer. However idk where they'd get their food from since they must import food and their biggest importer of food is USA. If there's an abundance of fertilizer from Russia they might be able to finagle a deal with several smaller countries for food.

Overall imo its not as clear cut if they got cut off since you have India, Russia, Nigeria, and a few smaller ones like El Salvador cutting off dollar and Euro collapsing. Everyone will be hurting and not in a position to follow US sanctions if they don't know where their next meal will come from.

I think you make a lot of good points except this one

They would collapse under their own weight in a year if cut off from the rest of the world.

What you really mean is cut off from the US Dollar. They would still have Russian oil and fertilizer. However idk where they'd get their food from since they must import food and their biggest importer of food is USA. If there's an abundance of fertilizer from Russia they might be able to finagle a deal with several smaller countries for food.