Time for us to wrap up and leave SK. We've been there 50 years. What's left to do ?
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We maintain a presence in ROK for a number of reasons, not all of them directly related to the defense of SK against NK.
NE PacRim is a vital region for US and Global interests. We need a platform from which we can project force quickly. If we abandon Korea then that leaves us only Okinawa (and they want us out).
The Koreas will unite, it's only a matter of time. If we are out of the picture physically then we can exert only minimal influence over what the unified country will look like. And, since nature (and politics) abhors a vacuum, our leaving will create a hole which China would likely fill.