Everybody knows these things become less effective after N years without an overhaul. At some point, it's not much deterrence if you assume the unmaintained war

Everybody knows these things become less effective after N years without an overhaul. At some point, it's not much deterrence if you assume the unmaintained warheads are unlikely to detonate and the unmaintained launch systems are unlikely to launch. We might not need to overhaul them, but in order to keep their deterrence, we need everybody to know we've overhauled them. If we actually do want to overhaul them, it's useful to do so relatively frequently so the knowledge and practices aren't totally lost.