Prescriptively, China has no historical reason to the territory enclosed by the nine-dash line. None of the preceding Chinese governments and dynasties ever held de jure or de facto sovereignty over that area. There's just no precedent for the claim.

Descriptively, if China wants the territory badly enough, it has a good chance of taking and keeping it, albeit at the cost of alienating the crap out of everyone else for at least a generation, if not more.

Realpolitik does not prescribe, but only describes.