If only America had done that...
They sell older versions. It is the same thing really. I don't know of any country that has any updated American equipment - though I am not an expert.

Also, they do downgrade some of their stuff (quite possibly by request from the receiving country that doesn't want to deal with depleted uranium). Like M1 Abrams that have been sold/given away.

Also nuclear states have... some kind of immunity against war for their territory.
So long as you have a leader that people believe is willing to use them in a small scale conflict. No one knows who the next leader of Russia will be. Do you think Russia would use nuclear weapons against China if there was a small scale clash on the Siberian border? I think Russia might respond to 20,000 Chinese troops with 20,000 of their own - at least initially.