This may be true, but I see the question as more of what people have a right to do, and not so much as hinging on whether they would be successful or not.
Many of the British didn't think the Colonies would be successful in their independence, but the Colonies asserted their right to do so even if it turned out to be a mistake.
My point here is that people have a right to do things that might turn out to be mistakes; that it is within their rights to make such mistakes.
Cuba wanted to be independent of the US, and so did the Philippines, and in both cases, I think they turned out worse for having become independent. They still had a right to become so, because this is what their people wanted.
If we had a right to leave the United Kingdom and form a Confederacy, then States had a right to leave the United States and
"to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness."
I'm also in agreement with you that a state is within its rights to secede, especially if its believes the rights of its citizenry and the state are being trampled by an immoral central government. Signing off on the Declaration of Independence and ratifying the Constitution were not a perpetual suicide pact once the country as a whole went off the rails. That is the definition of totalitarianism rather than a free association.
The “deal” between the central government and the states themselves should be a mutually beneficial one. Once it fails to be so, there should be a redress. Secession must be an option if it is an irreconcilable difference.
I will add (as well) that going by natural human rights, that slaves (slaves by heredity, not due to agreement they themselves signed, a la indentured servitude, which was a legal and moral agreement) also were worthy of the right to self-determination as well. They themselves were morally justified to utilize any manner and method to escape involuntary servitude. This was the double-edged sword that faced the South, yes the central government had their boot on their throats, but the South in turn had their boot on the throat of a substantial minority or (in the case of Mississippi and South Carolina) an outright majority of their own non-White population. They had the right to their own freedom and independence.