"Perpetual union" is unambiguous. Extending that to the Constitution through the "more perfect union" clause seems a stretch. There were problems with the AoC but I'm unaware the Constitutional Convention considered "perpetual" to be a form of imperfection that needed improving upon. The fact that Lincoln used the "more perfect union" clause to deny a right of secession tells me there was no stronger legal argument that he could have used. If a stronger argument existed, surely Lincoln would have used it. I can understand why the states agreed to a perpetual weak union but denied a perpetual strong union. In 1787, the founders could easily have said, "If these are the new rules, then we reserve the right to leave."