This debate comes back to OPCON transfer and can the ROK military defend their own country. First let me state up front that it is my belief that if the ROK and north Korea were to have a war without outside intervention or assistance the ROK would win and in the end win decisively. But both north and South would be devastated at the end and in fact things might look a lot like 1953 especially between Seoul and Pyongyang. And of course it is unrealistic to think that there could ever be an isolated conflict not the Peninsula, that does not spillover into other parts of Northeast Asia and does not have a crippling global economic impact. But if anyone can envision an isolated conflict then by all means plan accordingly.
However, I think the issue, for the US, should be about what is in the best interests of the US. I would submit that it is in US interests both to deter such a catastrophic conflict but if such a catastrophic conflict occurs (whether the result of war or collapse) it is in US interests to both end the conflict as rapidly as possible, minimize destruction on the peninsula and in the region (to include Japan as well though that pains many on the mainland to hear), ensure a successful outcome of such conflict and then achieve the strategic end state of the Alliance (stated in 2009 and reaffirmed by both Presidents in May 2013) and that is the unification of the Korean Peninsula (ideally peacefully but of course north Korea has a say in that condition). Working backward from that goal should guide the analysis as to how to best organize the combined military forces to achieve that end state.
Rather than make comments that belittle our alliance (which also betray a lack of understanding of both the alliance and the regional threat and context) we in the US should focus on what is in the best interests of the United States. If we decide that it is not in the best interests of the United States to ever contribute to achieving "a stable, secure, peaceful, economically vibrant, non-nuclear peninsula, reunified under a liberal constitutional form of government determined by the Korean people" then perhaps we need to completely re-evaluate our commitment to the Alliance. Talking about OPCON transfer is counter-productive and a distraction from the real issues which should be how best to organize the complementary military capabilities of the ROK and US to optimize their strengths and mitigate their weaknesses (yes the US military has weaknesses too when it comes to fighting on the Korean Peninsula) so that the military forces can most effectively contribute to achieving the alliance strategic end state.