Nobody ever said that the constitution can't be amended or that how it's interpreted doesn't change.
You made the idiotic claim that they don't determine the constitutionality of law. That's what they do.
Well it's not an idiotic claim if it's the truth. The law IS constitutional, as a matter of being passed into law by the Congress. The SCOTUS only rules whether or not the constitutional law violated someone's constitutional rights. If they find it did, we refer to the law as "unconstitutional" but that is actually a misnomer. The law can't be unconstitutional because it was passed into law by Congress. The law is invalid if the SCOTUS rules it violated someone's constitutional rights. However, SCOTUS can make a ruling and consequently, violate some other constitutional right. I've presented examples for this. A SCOTUS ruling does not mean that all has been set right with the world and is in accord with the Constitution.
Clearly, when SCOTUS ruled that slaves were property instead of human beings with rights... that violated the hell out of the Constitution. The same with their Korematsu decision upholding the interning of Japanese-Americans in WWII. The same with Plessy v. Ferguson. Such is also the argument with regard to Roe v. Wade. The SCOTUS doesn't always "get it right" and their rulings of "constitutionality" are not chiseled in stone.
Actually a Supreme Court ruling cannot by definition violate the Constitution.
The Supreme Court can be wrong- I think the Court was wrong in Citizen's United- but their opinion is still valid- and the reason why the only way to overturn Citizen's United is by a Constitutional Amendment is because the Supreme Court's ruling interprets the Constitution- and once it is interpreted- only another ruling- or a Constitutional Amendment can change it.
Again... what do we mean when we say "is constitutional"? Does it mean what SCOTUS currently, at the time, has ruled "is constitutional" or is it what actually adheres to the principles of the Constitution?.
Who determines what 'actually adheres to principles of the Constitution?
Sure you can think that State laws against mixed race marriage or same gender marriage were perfectly Constitutional- while others thought they were not. Some people thought state laws banning gun ownership were constitutional- and others felt such laws were not.
The Supreme Court is who determines what adheres to the principles of the Constitution- that is who is entrusted to make that decision in our system because there is ALWAYS disagreement about what is Constitutional.
We may not agree with the Courts decision- but it is always Constitutional at that time. Dred Scott was a horrible decision- but it was Constitutional.
So we changed the Constitution.
If Dred Scott was not Constitutional- we would have had no need to change the Constitution in regards to Dred Scott.
Well... Since Dred died in 1858, we didn't change the Constitution for his benefit. Seems to me I recall a rather massive Civil War that happened after Dred was dead, which was the impetus for change to the Constitution. Then, for the next 100 years, we "relied on SCOTUS to determine constitutionality" and had institutionalized segregation.
We can look back on some really awful SCOTUS rulings and wonder how such things could have been considered "in the spirit of constitutional principles" at all. In fact, in virtually every ruling there is a dissenting opinion which makes an argument against the ruling. This sometimes gives us insight into how a particular case was won or lost, what the other side of the coin was... because there is almost always other constitutional rights in play.
Very often, the SCOTUS is not simply deciding a case based on one specific constitutional right, it involves several and from various perspectives and parties. It would be impossible to have every law on the books never infringe on anyone's constitutional rights ever. The SCOTUS could never issue a ruling if they were charged with ensuring everyone's constitutional rights were respected. Very often, their ruling is a case where constitutional rights collide. You believe you have a right and I believe I have a right, and the SCOTUS determines which one of us has the more important right. It's not guaranteed they get it right.
Sure you can think that State laws against mixed race marriage or same gender marriage were perfectly Constitutional- while others thought they were not.
And SCOTUS rulings don't always resolve these issues. That is my point. Obviously, mixed race marriages were banned for many years after the 14th Amendment and I'm sure SCOTUS rejected many cases through the years contesting this. Some cases may have never made it that far, they were shot down at the appeals level and never reached SCOTUS. That is what makes cases like Dred Scott so important... it represents more than a single case that made it to the SCOTUS where the court made a ruling. It represents hundreds of cases that never made it that far, thousands of cases that were never brought, millions of people who were denied their constitutional rights for many years. It also illustrates how even the SCOTUS, with all it's wisdom and understanding of the Constitution, can still get it totally wrong.