Some decades ago, during discussion about test bans, the people who had experience from blowing up 1,000+ nuclear bombs were asked if it was possible to design a nuclear bomb that could be used by future generations that would be sure to work without additional testing, and bearing in mind changes in technology. Their answer was "yes".
(I think my knowledge comes from one of Freeman Dyson's accounts.)
I have not heard a clear explanation of why those earlier weapons designers were wrong, only concerns about how they might have been wrong. That more nebulous caution sounds identical to the sorts of statements someone might make as a justification for designing new weapons - for some people designing nuclear weapons and watching them go BOOM is a lot of fun.
In my very limited understanding of the topic, the largest unknown is the changes to the plutonium-gallium alloy of the core. Some worry that it has degraded. Others say "most plutonium pits have a credible lifetime of at least 100 years". Even if it's a problem, one solution is to re-smelt the cores, though currently we do not have the facilities for that. However, we could make those facilities without testing new weapons.
(I think my knowledge comes from one of Freeman Dyson's accounts.)
I have not heard a clear explanation of why those earlier weapons designers were wrong, only concerns about how they might have been wrong. That more nebulous caution sounds identical to the sorts of statements someone might make as a justification for designing new weapons - for some people designing nuclear weapons and watching them go BOOM is a lot of fun.
In my very limited understanding of the topic, the largest unknown is the changes to the plutonium-gallium alloy of the core. Some worry that it has degraded. Others say "most plutonium pits have a credible lifetime of at least 100 years". Even if it's a problem, one solution is to re-smelt the cores, though currently we do not have the facilities for that. However, we could make those facilities without testing new weapons.
See https://en.wikipedia.org/wiki/Reliable_Replacement_Warhead and http://depletedcranium.com/do-our-nuclear-weapons-work/ for some of the discussion. Also bear in mind the annoying calculus of game theory. A higher inability to use a nuclear weapon may make it less likely to be used.