Both secession of individual states and the abolition or replacement of the constitution itself are implicitly covered under the provisions governing the amendment of the constitution. Secession was, and still is possible. All that is needed is a constitutional amendment providing for the secession of the states that request it. Similarly, the present constitution could be replaced in its entirety through the amendment process. The amendment could be, in effect, a "deconstitution" detailing the disposition of federal assets and liabilities, treaty obligations, etc.
What is NOT provided for in the constitution is the unilateral secession of individual states.
It is an interesting "what if" question as to what would have happened if a constitutional amendment resolution were introduced in congress in 1860 allowing for the secession of the slave states.
Are you really that dumb? Did it ever occur to you that with the supermajority required by your thesis, you could have any policy you wanted? Why in the Hell would you secede if you could dictate to the minority any policy you wanted?
Nutty as it is, let's try out your theory: because of the difficulty in amending the Constitution, a situation could develop where 74% of the states wanted to secede, but the remaining 26% opposed it. Nobody in their right mind in the ratifying conventions would have voted to surrender that much sovereignty. Such a surrender would be not only to the Federal government, but to the whim of a small minority of other state legislatures. They were in their right minds, unlike your analysis.
All attempts to agrue secession as Constitutionally prohibited ultimately come down to needing Lincoln's hallucination that the Union preceded the Constitution. All else is window dressing.
Actually, the secession issue did need a Constitutional amendment. An amendment to prohibit it.