The average person expects the camera to simply recognize them by merely detecting one in the viewfinder, they don't expect to download an app or press a button

The average person expects the camera to simply recognize them by merely detecting one in the viewfinder, they don't expect to download an app or press a button to enable the feature. It's seamless on iOS and on many (but not all) Android camera app. Also, Lens is most likely uploading all sort of data directly to Google, not the best solution for a privacy-conscious person (I'm aware of the irony of privacy implication when talking of Android, but it's either that or the walled garden of Apple).