...so build quality is hard to quantify, and in most practical cases is subjective.
- Rubber breaks down faster than metal
- Rubber stains more than the metals you'd use in a phone
- Rubber can be torn, metal can't
So yes, objectively.
If you drop your phone a lot, rubber might help, but you'd need more than what the Fire phone provides.
No, it isn't objectively. You are subjectively choosing the criteria that makes one purportedly better than the other, yet they have zero applicability to the use or operation of a smartphone.
Grip matters very much with a smartphone. Rubber breaking down? Has anyone ever actually seen this happen? Is this actually a problem? Torn or stained? Again, is this actually a problem in the context of a smartphone? I would say no, it isn't at all.
A soft, comfortable, lightweight device that you don't have to immediately shroud in a protective case is, by actual use-based criteria, arguably the better "build quality". Build quality is suitability to purpose, not aesthetics or cult of material.
You have different criteria than robustness - understood, grip is important to you. Grip has its own objective measurements.
Christ.
"Build quality" is such a meaningless term in common usage of smartphones, and speaks to personal subjective aesthetics, not some utility of purpose.
And this is all particularly interesting given that there's a good chance a significant part of the new iPhones will be made with plastic. But I'm sure it'll be a "better" plastic.