The Nexus brand (and most of the devices that carry it) are like the equally-legendary HP LaserJet printers - take care of them, and they likely will outlast you.
Consider the Samsung Galaxy Nexus - it was the first Nexus device that ever ran ICS; shockingly, it never went beyond JellyBean in official service. So how come it is running Android 7.1 today?
Despite it BEING a Nexus, the older-than-mold GNex has "flaws" that would have supposedly mitigated against it getting upgraded beyond JB.
1. It doesn't support microSD. (Considering that Lollipop and later, by design, mitigate against the use of microSD as anything except auxilliary storage, this is a "flaw"? Further, the GNex has a crapton (for even today) of basic storage post-firmware - how many of today's non-flagship phones (or tablets, for that matter) need SD support due to being starved of basic storage?)
2. It's a dual-core. (So what? There is nothing in ANY post-JB build of Android that absolutely positively DEMANDS a quad-core (Unlegacy-Android 7.1 is hard data a-plenty there - in fact, the same can be said of CM13, for that matter). War Commander: Rogue Assault is, literally, THE newest game on Android to catch my interest - it also requires - at minimum, Marshmallow. I have both a Samsung S7 (which does have a quad-core CPU) and my GNex with the game installed; while the game DOES, admittedly, run better on the quad-core, it is not lagging THAT much on the GNex, despite the core deficit.)