For what its worth, The Court has warned Congress before that it should re-look at the formula, because it might not be constitutional, it gave them a chance, and Congress didn’t do anything with it.
Quite a few legal scholars agreed that the law as it was had little merit, and that striking it down was (in a strict legal sense) the only solution.

Personally, I do not agree with it, but I can clearly see the reasoning behind it, especially because it was enacted for another 25 years, meaning that Congress effectively stuck to the ’60s for 20 more years.
I do expect a flurry of lawsuits over re-districting in 1,23…