Considering the history of the time there was no thought of un-forming a nation (union) at the time the union was being formed. Indeed, the great compromise between those who feared a strong federal government (the anti-federalists view which many southerners supported) at the loss of state power and those who felt the nation could not survive without a strong federal government (the federalists) able to
unite the states when needed was through several negotiations. You must also
remember that it was the Virginia plan (Virginia, a southern state and one with significant prestige and power at that time and up through the Civil War era) is the one that fought for a strong federal government and the three branches of government (judicial, legislative, and executive) that ultimately gained the votes of all present and was formalized in the Constitution in 1787.
Beyond this, one must also recognize that even back then votes were purchased and/or given in an effort to obtain what the delegates felt would best serve their
interests in the short run. Therefore, the southern states and the northern states, arguing over taxation of slaves, import fees, the legal value of a slave in terms of whether they were or were not a full human being for taxation/property purposes, and many other factors led both north and south to compromise and agree that no stop to slave imports would be made for 20 years if the south allowed a simple majority vote to determine navigation laws (in effect, due to population density in
the NE states this meant the South lost power over navigation matters but they protected the slave trade so vital to them at the time. This ended any arguments over slavery at the time and allowed the U.S. Constitution to be drafted and then accepted by all states. You can read about that here
In the end, therefore, it was not that the southern states were stupid or ignorant in any way that no "out" clause was put into the Constitution. First, it would have been unthinkable for any state delegates, from anywhere in the recently formed U.S., so soon after the Revolutionary War, to create a document that would have, essentially, provided for the destruction of that union just as the union was being created. Second, as so often happens in human history, the northern and southern
delegates merely provided a short-term solution (slavery for 20 year for rights to control navigation (import fees, trade, etc.) necessary so that all states could accept the Constitution. Of course, they never dreamed that slavery would become so divisive, but at that time they could also not foresee that technology and the Industrial Revolution that so economically and technologically advantaged the northern states while southern states, so tied to agriculture, grew less able to provide
for their own military might as their technology did not gain from the Industrial Revolution. At that time both likely thought that in 20 years' time the issue would not still matter (people are often overly confident that their views will, in the end, prevail over those who oppose them). How often, in any government or during a time of
government legal writing do the questions "how will this turn out" and "what do we do then" get asked? Pretty much never, the only important thing is to get it done, to solve the current issue, and at that time the current issue was "how do we survive as a nation".
Also, the south ultimately left the union because of its belief in the need for state governments, not the federal government, to have the power to decide whether a state could or could not have legal slavery, not because they felt it their "natural right" due to any provision in the Constitution nor any discussion held during the Constitutional Convention. Slavery became the rallying point prior to the Civil War that completely split the federalist versus anti-federalist views, but when the nation was working to draft the Constitution those groups were arguing taxation, military protection, and other "national issues" while slavery was not seen as the key issue dividing them. To the south they were merely creating their own "revolution" to form a confederation with stronger state than federal power, that is what they viewed as their natural right, the same right exercised by the colonists when they demanded their freedom from England.