Slavery was "OK" - it was in the Constitution.

The Justices swear to uphold the Constitution, not uphold what they believe to be OK.

As unpopular as the decision in Dred Scott v. Sandford, 60 U.S. 393 (1857) was, as a matter of law the only significant legal flaw was the conclusion that any person descended from Africans, whether slave or free, is not a citizen of the United States, according to the Constitution.

The rest of the decision was soundly reasoned based on the law at that time.

But it does support the conclusion that the suggestion we put a "godless Vulcan" on the bench rather misses the issues.