This part is simply atrocious. I am using iPhone 5s just fine with latest version ios. Google should admit clearly on their online store that they do not have wherewithal to work with Network operators, hardware vendors to keep OS updated for at least 3-4 years.
I have an older iPad that ran smoothly and wonderfully when I got it but after taking enough system updates, it now run so slowly that I only use it as an ebook reader. Opening more than one or two browser tabs slows it to a crawl and I've found no real way to just flash an older system image to get it back to a more usable state.
At the same time, security updates are crucial so I do want to have those. The setup where I can get security patches every month or so but keep running a base OS version that the hardware can handle is ideal. Again, this isn't quite the same as the Nexus 5 is only 3 years old but it still runs surprisingly well with only major slowdown if I'm switching between lots of running apps due to memory limitations.
I guess my point is that in some cases, it's more important to get the core security updates (which Nexus has a fairly good track record of) than the latest bleeding edge OS features. May not be as good as a device that seamlessly runs the latest and greatest for 3-4 years but I find that after a point, you can't really use those newer features if your hardware is outdated so it's been less of an issue in practice for me.
The last supported OS version on a given iPhone version runs like a pig. iOS 9 is noticably slow on an iPhone 4s, and iOS 10 on an iphone 5-no-'s' is no exception, so I'm not sure how much that fifth year of support really counts for.
Alternatively, if I buy the Pixel on it's release date, is it safe to assume I will receive 3 years of major updates + 1 year of security updates?
Personally, I'd like to see Google switch to a BSD-based kernel, at least as an alternative option. Most of the interesting parts of Android live far above kernel space anyway.