If all the red states decided they didn’t want Biden on the ballot, they could decide who would be president before the election even took place. To prevent that from happening, it needs to be determined what the rules are and the legal precedent needs to be established for state courts to decide the legal battles.

This would require the state courts to all agree and then the Supreme Court to concur. Colorado isn’t getting the final word on who can be president, they’re dutifully interpreting the Constitution, making a ruling, and then letting the highest court in the land review it. It’s like the Maine case writ large, it needs to start somewhere, but just because that case started with a single person making a decision doesn’t mean that’s the whole process or that the process itself is illegitimate. There’s nothing unilateral about this, but the Supreme Court sounds like it wants to dodge responsibility for deciding whether Colorado is right or not by questioning whether they should have even been able to start the process.

Saying “Congress could just do the thing the plaintiff wants” is their tried and true dodge for things they know quite well can’t or won’t be done by Congress. They’re quite happy to implement law without Congress’s help when it’s something they want and quite happy to pretend they’re just deferring responsibility when it’s something they don’t.