MAD implies rational/peer actors. Some of the US moves in recent decades have been to deter and counter rogue nuclear states such as NK.
Also, it's not about rationality at all, North Korea has so far behaved perfectly rationally.
Multiple contracts were awarded for space-based missile tracking system that were specifically meant to deal with HGVs, so yes, there is another missile defense in the pipeline, that Pompeo literally called "SDI II".