"Necessary" is in the eye of the beholder. Stop trying to judge other people by what *YOU* regard as necessary. George III didn't think the Colonists reasons for secceeding were "necessary", but it was up to them, not him, to decide what was "necessary".
We today suffer under far more egregious taxation than they ever did, are we too conclude that we now have a better argument for independence than did they?
"Necessity" at least requires some material breach of contract that can be pointed to, and demonstrated true.
But no such breach existed in November 1860 when Deep South Fire Eaters began organizing for secession.
Indeed, the full government they elected in 1856 was still in power, nothing had changed.
That's why their declarations of secession were "at pleasure" and therefore not constitutionally legitimate.