[ubernostrum]

You are not quite correct. Article V places two restrictions on amendments.

The first restriction has now expired. It stated that prior to 1808, no amendment could give Congress the power to forbid the importation of slaves, and no amendment could establish or grant power to establish a direct ("capitation") tax.

The second restriction is still in force, but what it actually says is that "no state, without its consent, shall be deprived of its equal suffrage in the Senate".

So equal representation in the Senate could be abolished by an amendment, if every state ratified it (since that would satisfy the requirement of each state giving consent to "be deprived of its equal suffrage"). It's also arguable that the 17th Amendment -- which changed election of Senators to be by the people of each state rather than by the legislature of each state -- already violated this clause by causing the Senate to cease to represent the states.