This plan doesn't take complete control. It provides a mechanism for a web site to delegate trust on UA configuration authenticity to a third party, or even to itself via side-channel.

Nothing in the proposal requires the third party be Google. The proposal does decrease the control the user has over their own hardware, in the sense that it provides a channel for a site to decide the user-agent / hardware stack is the wrong pedigree to serve; that's not universally considered evil either (nobody really gets bent out of shape that you need a Nintendo Switch to use Nintendo Switch Online services).