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A glass back may prevent bending, and totally wouldn't introduce other problems
I think the main issue is the weight. They’re really trying to make it as thin and light as possible. Even now, the M4 13inch remains a bit too heavy to be handheld comfortably.
The use of glass might worsen this!
 
It feels like this would make the iPad heavier and more unwieldy to hold. If you desire more back support, perhaps get a magnetic back / folio to serve as extra structural support?
 
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