There are many good reasons to learn from cultures, to understand them, etc.
But cultural preservation is a means and not an end, unlike you seem to be arguing. When made into its own end, Cultural preservation for the sake of cultural preservation simply exists to build barriers and differences between people.
That is literally the purpose when it is its own goal. To build a shared thing among a group that others do not have, so you can divide your group from them.
Cultural preservation only unites people by pitting them against an out group.
I'm honestly unsure how you can argue otherwise.
It has been the cause of many great atrocities precisely because it always creates an out group.
So yes, preserving culture for the sake of preserving culture is a net negative for society.
The rest simply becomes an argument that nobody should ever have change. Good luck with that. Change comes for us all. None of us live long enough that we should get to try to force future generations to abide our way of thinking.