Yes, indeed, it is a bit weird that we have a sapphire case back and a plexy glass on the dial side...
My guess is that a plexy is too fragile for a case back: Do you imagine all the scratches you'd get when the watch is not on your wrist?
The more logical, and I still think, the best solution would be a solid case back.
But Panerai has become more horological and less toolwatch...
Best,
Nicolas.