After doing some digging and talking to some people, we can say
that it will be either very difficult if not completely
impossible for any phone that uses Qualcomm’s Snapdragon 800 or
801 to get an official, Google-sanctioned Nougat update (including
the Z3). And that’s a pretty big deal, since those two chips
powered practically every single Android flagship sold from late
2013 until late 2014 and a few more recent devices to boot.
This situation has far-reaching implications for the Android
ecosystem. And while it can be tempting to lay the blame at the
feet of any one company — Google for creating this update mess in
the first place, Qualcomm for failing to support older chipsets,
and the phone makers for failing to keep up with new software —
it’s really kind of everybody’s fault.
This is just how Android works. You shouldn’t expect your Android phone to ever get a major OS update. Instead, you get updates to Google Play Services. That sucks, but that’s just how it is, and almost certainly how it always will be.