The issue as I understand it is that's impossible, because there's no motivated stakeholder who can perform regression testing to validate the critical update c

The issue as I understand it is that's impossible, because there's no motivated stakeholder who can perform regression testing to validate the critical update channel. And the device builds have deviated far enough from vanilla Android+kernel that they require their own. Tbh, Google needs to work with the Linux groups pushing for more coherent ARM/device flexibility frameworks, then ban carrier and device manufacturer build modification below a certain level of abstraction. Otherwise they revoke Android branding + access to GApps. Then they would at least have a base for eventually saying, "We're going to enable a critical update channel where users take updates directly from Google. We will release these updates to you with a lead time in proportion to the severity. Unless the user has explicitly opted out, they will automatically receive the update after that period. If it breaks your phone, then users are going to stop trusting you as a manufacturer / carrier."