Yes, pretty much any statement about future human behavior is a guess. But I think the odds are well over 50%, likely over 90%, that a nuclear weapon will be deployed in the next 100 years. And I'd think the odds are 99.9% if we go out 500 years. Of course, as you point out, there are several potentially greater human extinction events if we are willing to extend the time horizon to 500 years.
The whole point of anti-proliferation is to keep that percentage as low as possible for as long as possible. If there were no risk of nuclear weapons being used as anything other than a deterrent, then there would be no need for anti-proliferation foreign policy.