What if people could prove that gerrymandering isn't legit, and shouldn't be legal?

Courts have rejected proposed districting maps on the grounds that the districts are unfairly gerrymandered, but of course there was no mathematical principle involved.

There have been proposals for mathematical criteria for unfair gerrymandering, such as looking at the ratio of perimeter to area of a district. But of course, no mathematical concept "proves" that gerrymandering is not legit, just as there is no mathematical proof that "majority rule" has to be the way to do things. A state legislature either agrees to a criterion and puts it into law, or it does not.

The trouble with gerrymandering is that both sides use it, so neither side is willing to give it up, even though they know it is wrong.