[throwaway101416]

There are such mechanisms.

Wire for example, doesn't require you to sign up with a phone number. You don't even have to have a phone.

How is it not an obfuscation mechanism that works at scale?

Also, what if google has a rogue employee who works for one of the surveillance agencies?

Trusting all your private data to an advertising company that has root access on your device in an app that is explicitly built around security and privacy is insane.

[fao_]

  Also, what if google has a rogue employee who works for one of the surveillance agencies?
  
  Trusting all your private data to an advertising company that has root access on your
  device in an app that is explicitly built around security and privacy is insane.

In that eventuallity they might be able to keylog or otherwise monitor the activity of applications anyway. I don't see how you can avoid that sort of eventuality aside from tossing the mobile phone and using snail mail and a OTP. Even then there could be someone looking over your shoulder.

[throwaway101416]

it would be much harder to implement such monitoring if they don't have root on your device.

There are degrees of difficulty. If you use Signal, you make it more difficult than if you use whatsapp but easier than if you have a rooted phone with open source software.

Also, you make it easier than it has to be with giving away contacts/phone numbers.