North Korea's military has been melting away as Pyongyang seeks bribe money in exchange for ending their nuclear programs. It is getting worse:
Many, if not most, North Korean army units have cancelled the majority of their training exercises this year, and shifted to growing food. This further reduces their combat capabilities.
North Korea needs nukes to really threaten South Korea. The rotting of the army makes nukes even more important to sustain a threat against South Korea to provide an incentive for the rich South Koreans to funnel money north. Which is why I don't believe North Korea will really give up all their nuke programs.
Yet if talks and a deal at least keep North Korean nukes limited, and America and Japan field real missile defenses, North Korea with or without nukes will find their only threat is their vast array of tube and rocket artillery looming over Seoul just across the DMZ. Using chemicals or just high explosives, North Korea could level Seoul in time.
If a crisis erupts on the Korean peninsula and the North Koreans fire even a warning barrage at Seoul, I expect the South Korean army to march north of the DMZ and carve out a no-launch zone in an arc around Seoul to protect their capital and home to a quarter of the population from North Korean artillery. And if the attack is focused just on a no-launch zone, will Pyongyang unleash nukes that might be shot down and which would trigger an American nuclear retaliation?
The North Korean army will soon be too weak to even defend their territory let alone conquer South Korea.