The ROK was built by the US, and modeled as they saw fit. You’re making the same argument that the US constitution doesn’t impact modern American life, because George Washington is dead. That’s a fallacy, it hasn’t been restructured in any meaningful capacity.
Yes, the ROK has peacetime control. They don’t have wartime control, despite posturing. The US still keeps the ROK on a leash, and is waiting for the time when they don’t even need to directly control the ROK as they will be subservient regardless.