The Supreme Court sole purpose was to decide defined disputes, not interject itself in Republicanism. States can roll over an play dead at the snap of the courts fingers whenever the rights they retained under their compact is disparaged, and they can also decide for themselves whether the court has invalidated their compact and ignore the court. They are certaintly under no obligation under the Constitution to give up their retained rights simply because the court says so.
Its like with this federal court ruling with Arizona punishing the state $500,000 a day because of aliens lack of english education funding. Arizonia is perfectly within its rights to say they are under no obligation to fund english only and that they are the final arbritrator in the matter under their constitutional comapct.
"They are certaintly under no obligation under the Constitution to give up their retained rights simply because the court says so"
Therein lies the real reason for personal income tax. Money is the real shift of power. Sure, a State can do what they want but risk losing Federal funding...No way.
Think about the states that either did not immediately go along with the 55 mph speed limit (or at least those that went along with the sign but didn't generate enough speeding tickets to satisfy the DOT). They lost highway funding. That was enough to bring everyone into compliance.