pdq1 - half the battle of one-upmanship is not so much the missiles themselves, as their ability to reach their tragets.
Upgrades on both sides arise because each side's defences become more sophisticated, therefore it requires more sophisticated (read expensive) to pemetrate those ever-upgraded erxpenses. The logic is that it's no use having a missle that can obliterate hundreds of miles of people and buildings (I know - how do people think like this?) if it can't reach its target, so upgrading is locked into the process.
It would be better if the leaders simply talked to each other, and dicussed differences, and put their money and resources into that rea, instead of sitting either side of the planet smug in the ability to obliterate each other.
It won;t happen in my lifetime, but having seen the changes in world attitudes that i have - I live in hope for my children and grandchildren.