Is there any reputable media source that you can cite to back this claim? China is on the rise but they have plenty of domestic problems of their own to deal with. Especially maintaining economic growth. 75% of their people are still living at poverty level and economic growth is slowing. I fail to see an interest in a forced take-over with an important trading partner such as South Korea. North Korea is in such a mess that China couldn't benefit economically from such a drastic measure.
South Korea's economy ranks 12th in the world. It's about the same size as Canada's and Russia's. It would be a disaster to the economic health of the world if South Korea were to go under. As you say, US interests are driven by it's own economic interests. To say the US has no interests in South Korea would be the same as saying it has no interests in Canada, also a huge trading partner with the US. Also both are military allies.
So, point to some links that I can read that provides background to what you claim.
Here is an article that says it's time to revisit the relationship with South Korea but nothing like what you say.
https://www.theatlantic.com/international/archive/2017/06/south-korea-alliance-north-korea-kim-moon-trump/532113/
A break in the US alliance with South Korea would not only destabilize the economy of an important trading partner but would bolster China's position in the area. Also relationships with other countries such as Japan and India would be weakened. A nuclear North Korea changes the game but not the relationships.
