A lot of Galaxy Nexus owners were disappointed to hear that even though KitKat has such low requirements in terms of hardware resources, making the Galaxy Nexus more than capable to run it, they will not be receiving anymore official upgrades for the Galaxy Nexus. That means they are stuck at Android 4.3.
I can't say this move surprised me, because Google did the same thing with the Nexus S, which only got upgrades for 18 months, and when the time for the 2nd Nexus after it came (Nexus 4), they dropped support for it, and left it at Android 4.1.
We also know that Google was trying to get the "Update Alliance" to update their devices for 18 months, too, which at the time I thought was quite low, and I wished they'd support them for at least 2 years. I think it makes sense to support a device for the entirety of its life cycle on contracts, and not drop support for it just before, because OEM's want to force you to buy a new phone.
Let's also not forget that even this relatively low amount of time for updates, 18 months, is really only delivered by Google for the Nexus devices, and Samsung (for its flagships only). There isn't anyone else out there who even matches that, let alone surpass it, even for their flagship devices.
That being said, there wasn't any serious reason why Google couldn't have upgraded the Galaxy Nexus, and it would've also been a perfect opportunity for them to showcase just how great KitKat is on older/less powerful hardware. After all, Google said KitKat is much more efficient, but didn't really showcase it to us in any way, and the Nexus 5 happens to have some of the most powerful hardware around.
ROM community to the rescue
Fortunately for the Galaxy Nexus users, while Google has abandoned them, the custom ROM developers haven't, and they've already shipped 2 working (alpha) ROMs (ROM1, ROM2). It seems the developers have taken upon the challenged to bring KitKat to the Galaxy Nexus as soon as possible, even if everything may not work properly from day one, but there should be a fully functional ROM soon enough.
It helps that it's still a Nexus phone, which is very easy to unlock and root, and Nexus phones tend to be bought by developers in general, which almost guarantees updates for a long time. So in the end Nexus phones are still some of the most upgraded Android devices around, while having a very high chance to get their life extended even beyond the 18 months of official upgrades, thanks to the developer community.