Secession makes no sense from a legal and constitutional perspective. A revolutionary one, yes, but not a legal one under the Constitutional instrument. If anything the Constitution shows the contract between parties, and parties cannot legally breach contracts. What is not included in an agreement is deemed excluded by contract concepts, and the right or ability to secede is not included in the agreement. Instead, they agree to unity and obligations to each other via a federal system, the documents contains things such as supremacy clauses, etc. It also makes no sense to form 'a more perfect union' by making it weaker than the Articles of Confederation that had already expressly disallowed secession as you mentioned by making the confederation 'perpetual.' Nothing could make a new nation/union more weak than no glue to bind it, and it stands to reason that there was no longer any need to state the obvious on the perpetual nature of the union. Moreover, a good number of the states were formed out of federal property afterwards--property that was purchased with federal funds in many cases--and to think that these federal creations are somehow superior in standing to the entity that owned and created them stands on its head.