9-dash line ambiguity might have been useful at one time, but it has passed the "use by" date. At the pace of China's economic development and military modernization, it no longer needs to 'own' the SCS to dominate it and have its own version of Monroe Doctrine. In fact, it should look at how US manages the Greater Caribbean to see how it's done.
I would tend to agree with you, but ultimately I don't know what China's actual end game for the SCS is to determine whether the strategic ambiguity is a worthwhile policy to continue with, so that's why I'm pretty much on the fence.