>That's an overly dramatic take. My prediction is that Republicans will now begin losing seats in red State legislatures precisely because of this decision.

Unlikely.

cf.:

https://www.cesifo.org/en/node/60300

https://www.washingtonpost.com/business/2018/11/13/least-thr...

https://www.evcnational.org/partisan-gerrymandering

https://changetherules.org/gerrymandering-united-states/