The US Constitution declares only one restriction on the amendment power, that no amendment can be made which would deprive any State of equal suffrage in the Senate without its consent. Otherwise, it would appear, the amendment power is absolute. While the idea of legal limits on the amendment power seems questionable, regardless of any legal limits, the amendment power was placed into the hands of the States so that it would be limited. Certainly 3/4 of the States would not use the amendment power to deprive 1/4 of the States of their lives/liberty/property. We trust that certain amendments would never be deemed proper by 3/4 of the States.
If there is a legal limit on the amendment process, it would seem to regard the ability to amend away the Union or the States. In Texas v White, the SCOTUS said that "The Constitution, in all its provisions, looks to an indestructible Union composed of indestructible States". Story said that the Union "is indissoluble through any steps contemplated by, or admissible under, [the Constitution's] provisions or on the principles on which it is based, and can only be overthrown by physical force effecting a revolution." Tucker said that a State would be annihilated if certain powers were not in its possession:
"But where a power of a State, included either in its reserved powers or its police power, is one without which no State can exist, which is essential and necessary to the protection of the health, the safety, or the morals of the people, such power cannot be surrendered by the State or absorbed by the Federal Government, because to do so would annihilate the State." Thomas M Tucker, Limitations on the Treaty-making Power Under the Constitution of the United States
Even if we accept the premise that the States cannot legally use the amendment power to delegate essential powers, it is hard to see how such a limitation could be enforced. If the SCOTUS were to have a power to declare an amendment to be unconstitutional, then there would seem to be no way to use the amendment power to restrain an out of control federal government. We trust that 3/4 of the States will not try to commit suicide by delegating away powers essential to their existance.
The SCOTUS seems to believe that the amendment power cannot destroy the Union or the States, which would seem to preserve our federal system. However, within this federal system, the SCOTUS has said that anything goes. Even if 3/4 of the States abolished the USBOR and created an absolute government, the SCOTUS has called this a peaceful and lawful revolution by ballot:
[i]The American Communists have imported the totalitarian organization's disciplines and techniques, notwithstanding the fact that this country offers them and other discontented elements a way to peaceful revolution by ballot. If they can persuade enough citizens, they may not only name new officials and inaugurate new policies, but, by amendment of the Constitution, they can abolish the Bill of Rights and set up an absolute government by legal methods. They are given liberties of speech, press and assembly to enable them to present to the people their proposals and propaganda for peaceful and lawful changes, however extreme. [/i] American Communications Assn. v. Douds (1950)
Again, we trust that 3/4 of the States would not abolish the USBOR and transform the US into a communist system with an absolute government. Even if they wanted a communist system with an absolute government, perhaps they would consider that Virginia did not consent to anything of that nature. It is one thing for 3/4 of the States to have their own revolution binding themselves, but it seems quite another for 3/4 of the States to have a revolution in the other 1/4 States.
In our system of government, the States have sovereignty against each other and against the federal government. If this is believed, if people see the states as having sovereignty against the other states and the federal government, then it would seem to follow that they would not use the amendment power as "peaceful revolution by ballot". All it takes is "1/4 plus one" of the States to have some minimal belief in state sovereignty to limit the amendment power from being abused in such a manner.
"I assert then, without hesitation, that we have created a Constitution of a democratic republic for the protection and perpetuation of popular rights, and on an assumption that whatever is done under it will be done in conformity with the principles which underlie that Constitution. Of nothing is that more especially true than of the provision respecting amendments. If the makers of the Constitution, in limiting this provision, stopped short of forbidding such changes as would be inharmonious, they did so because it was not in their thought that any such changes could for a moment be considered by congress or by the states as admissible" Thomas M. Tucker, Albany Law Journal, Volume 63