"The right to secede from the union shall be preserved for all states and persons."
This is an interesting idea. The Constitution has a mechanism for adding states, but it does not have any mechanism for removing states. Either something has to change or eventually the whole world will be part of the US (would that be a bad thing? Sometimes I think some problems would be solved if the US were to annex Mexico?).
Looking back at history, the Civil War could have been avoided along with all its bloodshed and destruction if Lincoln had been more open-minded about secession. The Confederacy would eventually have changed its tune on slavery because of cultural pressure or violent revolt of the downtrodden majority, and the Southern States would have rejoined the Union.
You can leave by moving out of the country, but are you allowed to leave the country by declaring the area in which you live no longer part of the country?