Nuclear warheads have a life.
Nuclear warhead designs have a life. Not all components are available forever. Try buying a 386 chip for example, or some more of those fancy capacitors you last ordered in 1991.
Nuclear components have a half-life.
The opposition gets better at opposing , or at least we convince ourselves they have.
Just like aircraft designs really, new or better ways of doing things are found. Otherwise we would still be flying the GR4 instead of the Typhoon, and we would not need NGAD/GCAP/ The French/ German thing.
N