I was thinking more on the lines of a unified Korea being a serious industrial competitor. After unification, I imagine the US military presence on the peninsula will decrease greatly.
I can't say I agree.

Firstly, I don't think the US military presence will change significantly for at least some years following reunification. It would take, at the least, a libertarian US regime or a populist Korean government catering to a fed-up population. I doubt either condition would be met for at least a while following. Well, I suppose I can offer one more: a new major foreign entanglement for the US.

Second, I see your point - China will reach an economic phase level with South Korea at some point, and once North Korea has been caught up, that nation's economic potential will be double what it might have been. The problem with this is that it attributes China's actions to a conspiracy with the goal of keeping Korea down for the next generation, which assumes the Chinese leadership really believe that they can prop the North up indefinitely. That can't be, as all well know that within some years there will be an economic and cultural breakpoint where popular unrest causes the regime to topple and pandemonium ensues. China knows that it needs a safer solution - it simply can't see an acceptable one, or seeks greater advantage leading up to it, so it prevaricates.