"China isn't worried about South Korea."

China is mostly "worried" about the US, then Japan, then it gets to worrying about South Korea. But I don't know if worry is much of a issue. Does "worry" explain why China is occupying Tibet?
China is most "worried" about China itself.

China does control N Korea and China had wanted to control all of Korea and it wasn't just all about Communism.
Some history:
"In 1392, the general Yi Seong-gye established the Joseon Dynasty (1392-1910) after a coup. King Sejong the Great (1418-1450) promulgated hangul, the Korean alphabet. Between 1592-1598, Japan invaded Korea, but was eventually repelled with the efforts by the Navy led by Admiral Yi Sun-sin, resistance armies, and Chinese aid. In the 1620s and 1630s, Joseon suffered invasions by the Manchu Qing Dynasty.

Beginning in the 1870s, Japan began to force Korea out of China's sphere of influence into its own. In 1895, Empress Myeongseong of Korea was assassinated by Japanese agents.[3] In 1905, Japan forced Korea to sign the Eulsa Treaty making Korea a protectorate, and in 1910 annexed Korea, although neither is considered to be legally valid.[4] Korean resistance to the Japanese occupation was manifested in the massive nonviolent March 1st Movement of 1919. Thereafter the Korean liberation movement, coordinated by the Provisional Government of the Republic of Korea in exile, was largely active in neighboring Manchuria, China and Siberia."

"In fact, China and S. Korea are very important trading partners. The Chinese are all about making money, and they don't make anything from the N. Koreans. Were the North Koreans stupid enough to ever attack the South, China would stay as far away from it as possible."

I don't see why trading partners means much, Japan and US are important trading partners as is Tiawan, as was Hong Kong.
China has through out most of history been the world's superpower and sees no reason why it wouldn't the world's superpower in the future. The reason China failed as a superpower is because it stopped trading- why would any rational Chinese want to repeat that mistake?

N Korea is useful to China as a buffer state. It doesn't particularly want to govern N Korea- then it would be responsible for it. N Korea functions as China's whore/garbage dump/scapegoat/proxy army/arms dealer ect.
In the future, China may want to sell it's whore and if offered enough money and they would probably do it now. But N Korean isn't much of a asset as it would be short term liability to S Korea.
So, S Korea generally speaking wants it's Korea back, the whore thinks it's tits or long legs are important, and China finds the current situation beneficial which includes as a trading chip [a carrot or stick]. And US policy generally ends up being something that amounts to offering flowers and candy to the whore.

If we leave S Korea, China may then decide that, it can now start a war with S Korea, using N Korea army as it's proxy. And there no reason to assume that China would not get involved in a war between N and S Korea- if the US left the area. It won't do it now, because US is there. China would *probably* abandon N Korea if N Korea provokes the US in a war.

Now, if S Korea want the US to leave the area, then I think we should go, but otherwise I think we should maintain a credible presense.