“The constitution must be adopted in toto and forever. It has so been adopted by the other states.” James Madison
Nowhere in the constitution does it say a sovereign state may not leave. In fact, in the federalist papers Madison specifically said a state is only bound by its voluntary act of ratifying the constitution.
Are those words also written in the Constitution? I don't recall seeing them.
In any event, Madison made that statement when the USA population was less than 4 million, and almost every American citizen could trace their ancestors to a half dozen countries in northern Europe.
Today, our population is 330 million, and barely half of them can trace their ancestry to northern Europe.
In 2020, the USA is un-governable.
We still exist only because of our exceptional productivity, and because a small majority of us are still guided by Judeo-Christian principles.