If that is the case, then either the phone should come with a case or it should not be marketed as a complete product.
How about making the phone more durable by adding 1 mm to its thickness, so that a 50 gram, 4 mm thick case won't need to be added.
I used my case not so much to protect my device, although it definitely does, but so I can hold onto the damn thing.
Without a case it's like holding the last gasps of a bar of almost flat soap. Also keeps it from sliding off surfaces.