Unlocking and flashing Androids is (in general) much easier today than it was 10 years ago, mostly due to Google's efforts with fastboot and other software infrastructure. Modern Pixels are really easy to hack on.

> in practice the vast majority of users end up running proprietary builds with big G's telemetry baked in

The vast majority of users are always going to do this, regardless of how easy it is to install a custom OS.