Great question! The Goldilocks principle still remains valid, and underscores a similar point made by many analysts today: the nuclear program is simply too valuable for the North Korean leadership to give up. Let me offer up a few more thoughts (mini-thread):
One proviso to keep in mind: my work explores how nations leverage the mere latent potential to produce nuclear weapons as an instrument of coercion. I focus on when nuclear technology - not operational nuclear forces - provides the most coercive leverage: dx.doi.org/10.1080/096364… 
Since North Korea crossed the weapons threshold during the Six Party Talks, I would point you toward the extensive lit about relative bargaining power among nuclear weapon states and the logic of arms control.
In this context, I did apply the Goldilocks principle to shed light on past attempts by Pyongyang to 'build, bargain, and pause' during the 2012-2016 time frame: dropbox.com/s/sqvq8u9wu3q9…
But I'll admit that this chapter advances the rather obvious conclusion that the North Korean program generated increasing returns over time as an operational capability but diminishing returns as a bargaining chip.
