And when would you have liked the decision handed down? Before it happened? The Supreme Court rules on cases as they come before them regardless of how long it takes. The ruling in Scott v. Sanford, the Dred Scott decision, was issued in 1857, ruling on something that had happened 11 years before. That's the way it works. The court can't issue a decision on something unless it is brought to them.
My point, which you must concede in fairness, is that you cannot bind people to rulings made after the fact. That's judging people a posteriori, applying the standards of one time to another. And the Court's ruling was teleological, in that, in 1869, they could only rule one way anyway, politically, because Lincoln had changed the Constitution with two million bayonets and revolutionized the basis of the Union by winning his political war against the South.
And it would have upset too many applecarts for them to have ruled correctly, to-wit, that the Southern States were within their rights, sitting in convention as The People, to have seceded from the Union.
You might say that the Supreme Court's decision was a necessary one, to perfume spoils of conquest reeking of the stink of blood. And its timing falls squarely in the middle of the period when the South in no way was a participant in the Union, but a conquest under it.