Even worse, you need people who:

* Care about the Android version on their devices

* Would pay reasonable rates

* Stick with an older phone / don't buy Nexuses

It's not as bleak as that. There are a lot of people with manufacturer-abandoned phones that would probably be willing to shell out a modest sum to get less ancient firmware. As with everything, the economics are a question of quantity and effort. A billion Android devices enter the marketplace each year.

I think the real problem is that it is just turning out to be difficult to make a silky smooth upgrade process, the kind that is consumer-friendly enough to create a mass market. It all fundamentally starts with "hack the device" and most people can't. Maybe it's just a really hard problem because it varies from device to device and base OS version to base OS version.

What's so special about Nexuses in this regard? I find the duration of support still pitifully short on them even if it's slightly better than what some other phones manage. Whether or not people care about their version, everyone should if only because of security.