There's a lot of supposition in your post and no evidence in favor. First of all what makes you think Gmail stores user data in plain text at rest? Secondly t

There's a lot of supposition in your post and no evidence in favor. First of all what makes you think Gmail stores user data in plain text at rest? Secondly think about your definition of "backbone" networks. Google's own network is extremely large in scope, and generally user traffic lands in Google's network at a point very close to the user. For example if a user in Japan sends mail to Google, that connection will be terminated in Japan, and backhauled across Google's network to wherever the actual servers are located. Google intentionally shrinks the scope of network surveillance by minimizing the network distance between the user and Google's frontends.