Some phones have fallen off the upgrade cycle due to Qualcomm discontinuing SoC support. I think one of the Nexii fell foul of that. Without HAL updates, OEMs are dead in the water. Last I saw a "Google-to-OEM" workflow, it said OEM get a hardware-support-pack one (maybe two) weeks before a new Android release. That's to get the kernel up.
Subsequent to the release, it's another week or so before the AOSP source dump. At that point the OEMs get to figure out what's changed/new/gone and add in stuff like dual-SIM support and whatnot. Lots of pain ensues if Google had done major work, i.e. Marshmallow's Doze seems to have clashed big time with Sony's Stamina mode (a major differentiator for them) and they've had to do major rework on that. Likewise their Smart devices/gear APIs, when Google introduced their own.
In a real sense, there's actually a penalty for those innovating/differentiating.
PS: D'oh because it's the closest to #facepalm