It seems to me that the easiest way that Google could do this would be to create a "Perfect Android" branding or similar, which has some tight rules around lice

It seems to me that the easiest way that Google could do this would be to create a "Perfect Android" branding or similar, which has some tight rules around licensing - to qualify, a device would have to meet a set of minimum hardware specs and the manufacturer and carriers would have to sign up to providing timely updates to the latest version that Google produces over the next N years. Google, for its part, would guarantee that those newer versions for the next N years will run well on the minimum hardware specs agreed to.