A plastic housing is IMO way better than glass. The screen is the most delicate piece of a smartphone because here glass is a necessary evil - and we have to add protectors and cases with raised edges to protect it - now you want to add a glass back...!!!? Here I can see once again opinion being driven by fashion rather than utility. Glass is only "better" than plastic where it makes sense to use it. If my phone was a sculpture I'd say yes to glass. Otherwise keep it far from me!
This attitude (fashion over utility) shot down the Pixel 4 (for me and pretty much EVERY online review): it was given a tiny battery apparently to make it look svelte and feel slim in the hand...! As as result I've been holding out for the 5 and am quite relieved to hear about the plastic casing - though the small battery life rumor is disappointing. Pixel engineers, how about this for an idea: make your battery at least the same size as the competition and then use the power saving features you tout to make it last even longer than the competition! Mind blown!