How does MAD theory work when everybody has nukes?

There are those who say America should learn to live with Norks having nukes.  But that would all but guarantee then that proliferation will get far, far worse.  If that treaty isn't going to be enforced, and there is a non-proliferation treaty that could be enforced, then where do you go next to deal with this threat?

What comes after MAD?

It was expected that Mutual Assured Destruction would deter any attack.  But that was in a world with few nuclear states.  If there is now to be any number of these nuclearized states, then what guarantees that a North Korea type failed state wouldn't attack if some third party agrees to rebuild their nation after it is attacked?  In this way, North Korea could attack and be attacked, and gain from it.  The damage to the USA would be far worse than North Korea.  Would you trade two American cities for two North Korean cities?  That exchange does not favor the USA.  It would incentivize the Russians or Chinese to offer assistance to North Korea if North Korea would nuke the USA cities and grievously damage it.  But what happens to the USA's interests?

It is not such a simple question as whether or not North Korea can keep its nukes.  The USA has already been attacked with asymmetrical tactics on 9-11-2001.  Such could happen again if there is a nation or nations willing to sacrifice millions of lives for the chance of destroying the credibility of the USA.  Iran has already made such a threat with respect to Israel.

So MAD cannot really work if an aggressor feels that has nothing to lose, and everything to gain by trying it.