I ask "Why save physical books?"
If they are truly rare, then they are likely not valuable, otherwise there would be more copies or their contents could be found elsewhere.
Owning and storing physical books is not free, there is a real cost. Even owning and storing the scans is not free, especially when IP rights and challenges get involved, since a scanned book with no distribution is worthless.
I disagree with putting books on a pedestal and saying they must be protected. If they were any good, they would stand on their own, but if we have to run a moral crusade to save them, perhaps we are all better off if they're destroyed.