I was exaggerating, but the demographic slump that happened as a result of WW1 was a problem for french military planners, which is why a large portion of their army were reservists or badly trained.
Per your other point, you're shifting the terrain of the discussion. WW1 was almost a pure losers war, and yet, most of the same states engaged in WW2. That's the crazy decision. WW2, on the other hand, had relative winners (although I think the US would have done well regardless, and the USSR got to be a global superpower (which it was already) at an appalling cost).
Another thing that bothers me about the whole 'MAD prevented the cold war from going hot' is that none of the generals at the time thought like that. People didn't really start thinking about nuclear weapons as anything other than just big bombs until well into the cold war. That's why MacArthur advocated their use in China.