USSR is not Russia. Afghanistan was a not a "core interest" or existential threat to any of the USSR nations.
If we use the Afghanistan analogy, Ukraine is on the border of NATO too. Interestingly, then NATO would be the overextended multinational entity trying to expand even further in this scenario. If we expect an "Afghanistan" scenario to repeat itself, then logically one would expect NATO to collapse.
In earlier comment you said: "Vietnam and Afghanistan .... Those were pure proxy wars. This is not a proxy war, it is a direct hot war."
And here you wrote: "This is not a proxy war. Russia is involved directly on its border. A proxy war is in a far off land." And I was saying that Afghanistan was not a far off land, it was on the border, so your two usages of "proxy war" weren't consistent.
Sorry, I don't understand your second paragraph, I was not trying to draw an analogy with USSR and Afghanistan.
As I understand, your argument is instead that: "Afghanistan was not important to USSR, so USSR did not consider using nuclear weapons then", while "Ukraine is a core interest of Russia, so it will rather start a nuclear war (and risk destruction of the world) rather than back down and return to the positions before Feb 24th."
I don't know if this statement is as obviously correct as it may seem to you. Certainly many believe that Russia is not willing to self-destruct to stand the ground here.
If we use the Afghanistan analogy, Ukraine is on the border of NATO too. Interestingly, then NATO would be the overextended multinational entity trying to expand even further in this scenario. If we expect an "Afghanistan" scenario to repeat itself, then logically one would expect NATO to collapse.