The rebuttal to that is easy. We have a form of government where the judiciary can change the constitution at will. Not by literally changing the words in the constitution, but by changing what they mean and how they will be applied to the public, by force of violence up to death, if that is what it takes.
The legitimacy of the judiciary is measured by its ability to obtain coercive power from the other branches. If Congress wants a certain species of guns removed from civilian ownership, and the executive and courts agree, then they'll kill, if that's what it takes to maintain their power. "Legitimacy" is just another word for superior force.
We do live in a constitutional republic. Without the constitution, and what your are suggesting, we would only have the trappings of a republic where "might makes right". Legitimacy is dependent on adherence to the constitution and it is not synonymous with superior forces. Regardless our government doesn't even have close to superior forces over the people. They have only done what we have allowed them to do and we need to start holding them to the constitutional standard. Not the one they make up on the fly but the written one.