What's missing from this is China making a lot of soft calls to SK, basically saying 'we like you, we can pressure NK into a more peaceful position, invest with us, we're all Asians', and SK seems to be responding positively.
SK being closer to China would be a game changer for geopolitics, and they would need _very strong_ guarantees from China, so strong I don't think it's possible in the short term (unless NK has a regime change).