I don't think that's a consensus view? Jeffrey Lewis says the opposite,
- "The point is that North Korea is clearly aimed at overwhelming the US missile defense system in Alaska... At that cost, I am pretty sure North Korea can add warheads faster than we can add interceptors."
> even if they got one through, wouldn't North Korea basically be a wasteland 30 minutes later
The idea is if to have a credible threat to keep us from acting first, as well as giving many countries a vested interest in the stability of the regime: if the Kims go, North Korea goes, and so does somebody else.
Congratz, you grasped the facet of MAD that is unacceptable losses - for the US a single warhead getting through is too high a price to destroy pretty much anyone.
We somewhat intentionally lack defences against MAD salvos. Our defenses will likely remain adequate for defending against the likes of Pyongyang.