Re: Holding action - wrong
To update the full Android system on a phone needs a build of Android customized for the hardware on the phone - for example the kernel is different on a phone with a Snapdragon chip compared to on with a MediaTek chip. To produce a full upgrade for a phone needs knowledge of what hardware is in the phone (and in many cases the private key of the manufacturer to allow the upgrade to be permitted). Google provides the sources for the kernel upgrades but cannot compel the manufacturers to implement them.
Google has moved as much of the Android system as possible into a separate blob that it can update because the manufacturers were not updating the bits they had control over.