Even red states are not made up entirely of Republicans. If it can be proven that partisan redistricting is un-constitutional at a state level then lawsuits, ballot initiatives, and general public grumbling can get districts redrawn.
"Gerrymandering" in and of itself, is a shameful old tactic that really has no place in this century. The Supremes didn't say it was a good thing, they just said it wasn't an un-Constitutional thing.
It could probably be constitutionally legislated into illegality though.