you do realize that China is the only reason North Korea is still operating, right? The North Korea that threatens your home country and brutalizes many of your relatives in North Korea?
See, this is what I mean when I say I object to such reasoning. People are so bold, they're always willing to sacrifice other people's lives to defend freedom.
But you can't sell prospect of a nuclear war against China, because people have iPhones made in China, so they know it's absurd: why would China want a nuclear war? A threat has to be believable to sell to people. So we talk about North Korea. Nobody in the US has bought anything from North Korea: they are a dark, evil, and mysterious people, and it's easy to believe they can be also suicidal.
(Yes, the North Korean regime is evil: they're so evil that they make China look like boy scouts. But they aren't suicidal.)
There are other reasons why going to war against China wouldn't be that advisable.
http://www.businessinsider.com/navy-chinese-microchips-weapo...
https://www.wired.com/2011/11/counterfeit-missile-defense/
http://www.cbc.ca/news/politics/fake-parts-in-hercules-aircr...
Alliances are double-edged swords. I'm curious, what would be your perspective if the U.S. decided that South Korea was no longer worth defending and pulled it's troops back to Guam and Okinawa?
You mean, what if the US decided that keeping an air force base within 1000 km of Beijing isn't worth the cost?
Well, it that case, it sucks, because our national defense budget will likely have to increase a lot. But an independent country cannot outsource its defense to others forever, so South Korea should have contingency plans for such a case, or at least I hope so.
As you said, alliance goes both ways. The military ties to the US did help us tremendously in the past. But the US is not stationing thousands of soldiers out of goodwill: they're doing it because South Korea is located at a really convenient position in America's game of global dominance. I think it's mutually beneficial (for now), but if America decides it isn't, well then that's it.
Hint: maybe we keep military bases in SK to defend South Korean, because over the past 67 years of our alliance, South Korea has become a fantastic trade partner and we want to ensure that such a mutually beneficial relationship continues far into the future.
South Korea's forces had lost 70,000 men in five days before the USA came to your aid in 1950. In the fighting that ensued, 37,000 Americans perished to defend your country. Please consider that.