If the Supreme Court legitimately does its job, then what it says the law is, and what the law actually is, will be one and the same. Nothing in the Constitution, however, states that Supreme Court decisions are superior to any other form of law, and nothing even hits that they should be considered binding upon people other than the actual parties to the cases before them. While the Constitution was designed to build a government on a Common Law framework, the supremacy clause suggests that the Constitution is supposed to be superior to judicial precedent.
To be sure, the fact that the Constitution says it is the Supreme Law of the Land doesn't magically make it so, and someone who does not regard the Constitution is the Supreme Law of the Land could perfectly well regard unconstitutional things as legitimate. I would posit, however, that if one regards the Supremacy Clause of the Constitution as true, then judicial decisions contrary to the Constitution must be recognized as illegitimate. Further, I would posit that if one does not accept the Supremacy Clause of the Constitution as true, then nothing in the Constitution has any meaning with regard to government legitimacy or lack thereof.
True, but aren't laws that DO affect the rest of us use, as precedent, narrow SC decisions?