Actually a good question. The physical weapons are old, but we contract out to defense companies to keep updating the missiles they are deployed on etc. You usually see an article every six months about our ICBM test that launches from Edwards AF Base. The nuclear enterprise includes a lot of laboratories and purpose-specific sites. A lot of those are legacy sites that are not used anymore because of treaties, but are still maintained by the government just because/in case, and that's probably a lot of the old tech you're thinking of. The laboratories that are part of the DOE, especially the ones with annual budgets over a billion dollars have fantastic equipment. Those are like modern NASA labs, where all of that research money develops breakthroughs in energy and other tech that is then handed off to the economy. The Comprehensive Nuclear Test Ban Treaty stops a lot of tech from being upgraded if it is too closely tied with the physical weapons because the weapons themselves can't be upgraded. I think low-moving-parts of old tech is an advantage though. Old school, locally controlled, physical systems can't be hacked.