Your primary argument is the one the Court used: the difficulty of establishing an objective standard for determining whether a map is fair. But in fact there is a ridiculously simple and obvious standard: the division of seats between parties should reflect the division of voters in the state. In the example given, we had roughly equal numbers of voters from each party, yet 10 out of 13 seats went to Republicans. The obvious standard would be to require that each party get either 6 or 7 seats. What’s so difficult to understand about that?