Is your theory that nuking the USSR would have completely prevented nuclear proliferation for at least two centuries? That no other nation would have developed nukes in secret, or stolen some from the USA? Or is your theory that nuking China, India, Pakistan, France, Israel, etc., as soon as they started to get nuclear weapons would still be worth it?

If you're saying global thermonuclear war is a bigger risk than we tend to think, e.g. because of anthropic shadow effects, I agree. But I still don't think there's positive expected value in a pre-emptive all-out nuclear attack.