Well, the mainstream adoption of plastics following World War II happens to coincide with an unprecedented period of relative global stability known as the “Long Peace.” So there’s at least some version of “thriving” that was, in fact, not previously achieved. Correlation does not imply causation but in this case it’s difficult to disentangle which innovations have lead to the relative global prosperity that has enabled this, and I think plastics are on the shortlist.
You are mistaking plastics for the MAD doctrine, and nuclear weapons.
The classic US take on global peace, ask the Koreans, Vietnamese, Iraqis, Afghans, etc, how they feel about the long peace?
Moreover, pretty sure it was nuclear weapons, not plastic, that stopped the imperial powers from directly attacking one another.