North Korea is geopolitically useful as a buffer state between the United States' sphere of influence in South Korea, and China. China has defended it pretty determinedly, historically.
You don't even need China. They have the Hwasong-17 and nukes and would very likely use them to retaliate. There's a reason they were quite keen to develop nuclear weapons and missiles capable of delivering them to the US mainland.
Even short of a nuclear response Seoul is in range of conventional artillery from North Korea and is dug in enough you couldn't destroy them all before the were able to do significant damage to Seoul.
Even short of a nuclear response Seoul is in range of conventional artillery from North Korea and is dug in enough you couldn't destroy them all before the were able to do significant damage to Seoul.
[0] https://missilethreat.csis.org/missile/hwasong-17/