As dumb as it sounds, mutually assured destruction does have the perk of keeping everyone from using nukes. If modern countermeasures prevent that, it isnt a deterrent anymore. Updating these nukes improves the likelihood we dont have to use them.
Relevant example: russias tanks. They are outdated and weren’t adequately improved over decades. and are now getting wrecked by consumer grade drones and guys with while fancy, in all honesty, second grade hand me down rocket launchers. Before we knew this fact, they were a reasonable deterrent to not fucking with russia. Now, not so much.