At some point the past has nothing to teach anymore.

Perhaps not the painting. Perhaps we develop a technique which can learn from the dust on the ground, or the sediments accumulated in the cave.

It is a very small cost to keep the cave closed. And in the history of humankind we have multiple times destroyed stuff later generations could have learned from.

Just look up for example the history of the Herculaneum papyri[1] and all the attempts to unroll them with destructive methods. Many of those were destroyed forever without being read or only partially read. And it very much seems like we are on the precipice of having the tech to read them non-destructively.[2]

And if all you want is to ogle at the paintings then you can do that in the replica cave?

1: https://en.wikipedia.org/wiki/Herculaneum_papyri 2: https://scrollprize.org/

The book directly pulls on the kind of data on early societies that the scientists studying these caves produce.