I think that really is a great point. If Google actually sold their phones for more than 24 months, they'd stop supporting software updates before they stopped

I think that really is a great point. If Google actually sold their phones for more than 24 months, they'd stop supporting software updates before they stopped selling brand new phones! Would you buy a Dell, HP or Lenovo today if Windows 10 would only get software updates two years after it was released (July 29, 2015...)? To be sure, phones (and computers) don't need software (feature) updates, necessarily. They can still perform functions (and install apps) long after the updates stop coming. Security updates, on the other hand, should have a guarantee more inline with computers - say 5 years after the product is no longer sold. (Of course, that's much more ambiguous then "2 years after release date" but 5 years for security doesn't seem unreasonable to me.)