If you try to make the Constitution unamendable, you guarantee that the Republic that is founded on the Constitution will eventually fail.
I can't think of a worse idea for the long-term health of the Republic.
Do you want to be in a US without the Fourteenth Amendment?
Actually it strictly immutable without any of the Amendments, no 1st, 2nd etc.
That it's been done numerous times in the past suggests it is quite far from immutable.
If you try to make the Constitution unamendable, you guarantee that the Republic that is founded on the Constitution will eventually fail.
I can't think of a worse idea for the long-term health of the Republic.