Google has moved a lot of capability from the core operating system to updatable libraries. For example, the system browser isn't linked to the OS. So if your Android version is a few versions behind you're generally not missing too many capabilities, nor do you run into too much app incompatibility. So if the vendor only updates the core OS for 3 years, it's still generally quite usable for a few years more. Sure, it'd be better if the core OS updated too, but IMO it's not a major annoyance.