[4CJ]

All ratified the Constitution as it stood, and that included the procedures for adding new states to the Union.

True. But of the 1st 13, No state could have refused to accept the ratifications - thre was no congress under the new government. And the Constitution is SILENT on the process of leaving, and whether or not it requires consent.

[Non-Sequitur]

And the Constitution is SILENT on the process of leaving, and whether or not it requires consent.

True, but since the Constitution required the consent of the other states to join the Union, and required the consent of the other states to combine or split up or change borders by a fraction of an inch, the it makes more sense to infer that consent of the other states would be required to leave as well.