El Guapo wrote:
(1) The Constitution provided for its alteration (which would presumably include its termination or replacement) via amendment and/or new constitutional convention. That would be the replacement for the clause you mention in the Articles of Confederation - that's the process if you want to end the Union. And there being no termination / expiration date is the same thing as it being perpetual (until terminated / changed).
(2) My theory is not at all that the South should bend to the will of the North. There are (and have never been) no such legal entities as either "the South" or "the North". As such there never was "partnership" between North and South, it was a Union formed by the people of the states. The North and the South are (non-formal/legal) regions within the United States, subject to the Constitution of the United States.
I am saying that all states within the United States (including the Southern States) are obligated to adhere to the Constitution of the United States. It sucked for the southern states that population growth post adoption of the Constitution went against them and increasingly deprived them of their ability to (as a group) vote down anti-slavery laws. That is legally irrelevant to the obligations of the states under the Constitution, however.
All states are obliged to bend to the will of the voters of the United States, in accordance with the terms of the Constitution.
Then you're farther off than I thought. The only voters of the United States are the States. It isn't a democracy. It has never been.