Well said, but how do we know when such miniaturization is complete, if it has not already been accomplished.
Modern thermonuclear design is very well know, and assembly is rather easy, once you have the Pu and the Tritium for the zipper.
Our problem is if he demos one with a mid-Pacific detonation, on an arched high altitude shot, and then says, oh, by the way, I have 5 more all on easily-hidden mobile TELs, and they are aimed at New York, DC, LA, London, and another city I will not tell you about.
Now what do we do, because he could launch before we could destroy those 5 missiles?
That is the real problem.
We already have several developed, deployed, and tested ABM systems (Patriot, THAAD, SM-3, and the Alaska-based exoatmospheric interceptor missiles whose program name escapes me at the moment) which can deal successfully with a limited numbers raid like the one you postulate.
The recent successful ABM tests conducted by the Missile Defense Agency (MDA) were, among other objectives, meant to underscore the existence of this capability to outside parties, especially hostile ones like North Korea and Iran.
Along with the MDA, a big thank you is owed to President Reagan and all subsequent presidents (both Democratic and Republican) and Congresses for their support in developing this capability. It took a long time and a lot of money to accomplish it.
This is why the question of what to do once you have defeated a limited nuclear ballistic missile attack is salient and why I think the Chinese would act pre-emptively. They don’t want to find out how well or poorly their ABM defenses would perform when dozens of US nuclear missiles in multiple waves come arcing over the Pacific inbound for North Korea.