A few years ago, when the 4/4S were still sold, there were videos and articles about Apple and "planned obsolescence" making the rounds over social networks like Facebook. A few people I know referred to these and mentioned the glass back as an example of planned obsolescence, like Apple put it intentionally to make the phone easier to break. (Replacing the back was only $29 though).
As much as I loved the 4/4S design, it is true that it makes the phone more fragile. But apparently in 2017 it's not a concern to people anymore (I guess because Samsung switched to glass backs it's ok now?)
Or because there have been three generations of advanced glass since then.
https://www.corning.com/gorillaglass/worldwide/en.html