In practice, carriers and manufacturers do not tend to release updates for Android phones other than Nexus once the shinier model has come out a few months later.
One of the major selling points of the iPhone is that it will continue to receive full OS updates for 2+ years.
Continuing software upgrades are pretty Apple-specific in the smartphone market, but it's also just as true of Windows and Linux in the desktop space.
It's basically Apple's model with iPhone and Macs, except applied to automobiles.