There is actually nothing in the Constitution which forbids a State from leaving the Union either.  It might be said then that the Constitution is silent on the matter and, being silent, cannot be used to prevent a State from leaving.  You also forget that under Article 5 2/3 of the States themselves may call for a convention to amend the Constitution, 3/4 is the figure for ratification. And it isn't so far fetched that an entity of their creation, the federal government, would be dissolved by them should they come to the conclusion that their security and happiness would be enhanced by dissolution.  The amendment could simply read that all grants of power from the States to the general government are revoked effective with the adoption of the amendment.  Bye bye federal government.  Still want to entertain us with your fantasy that the States are not sovereigns?