(which would guarantee permanent slavery)
“No amendment shall be made to the Constitution which will authorize or give to Congress the power to abolish or interfere, within any State, with the domestic institutions thereof, including that of persons held to labor or service by the laws of said”
the amendment only restricts the actions of the Congress of the United States. It does not prevent any state in the Union from taking action to end slavery in that state. Nor does it restrict the Congress of the United States from prohibiting slavery in any territory of the United States. So the amendment does not guarantee “permanent slavery” within the United States.
That's wishful thinking. I do not think you honestly believe that amendment would not have kept slavery going in the USA until it was no longer economically viable.
I try to be honest with myself, and I think you do too.
But why would the Northern states support such a measure?