The same can be said of Supreme Court decisions -- since none of the other divisions of power (president, congress, people) assert majoritarianly that the Supreme Court can't interpret the Constitution. I know many individuals and philosophies argue that it isn't even up to the Supreme Court to interpret the Constitution, but that's not the majoritarian view of the branches of government/people. So while they can continue to advocate that position -- it is illogical to say the Court itself is acting in an unconstitutional manner.
Again, that's why Libertarians hinge their principles on something more objective than the Constitution -- as useful as a good Constitution can be.
An amendment which made the number of Senators from each state variable (based on population, GNP, number of movie stars in residence, or any other basis) would clearly be void from its inception unless either (1) it was ratified unanimously, or (2) every state which did not ratify it would have at least 1/(number of states) of the Senators unless or until such time as they ratified it.
The existing Seventeenth Amendment is perhaps questionable for much the same reason, since there are IIRC three states that were present at its ratification but have not ratified it themselves (any state that ratified the Seventeenth presumably consented to it; likewise any state that joined the Union after the Seventeenth was in force). Of course, even if the legislatures of those three states were found to have the power to appoint Senators themselves independent of the popular vote, I doubt any of them would want to do so.
Articles in Amendment to the US Constitution were enumerations of what the Fed Gov. could not do. They were not intended to be uo so the Fed Gov. could give themselves additonal power. Notice the little clause behind most of them how the Congress gives itself the power to enforce the later amendments. In fact, most of the Amenedments past the 11th are illegitimate. sui