This is a great point and one that so many people miss. Time spent managing email is not necessarily time wasted. Working with a team involves communication, and often that communication happens via email, and so managing email can be productive. When I'm in the midst of coding, I'd much rather my teammates send me emails that I can handle asynchronously than have them stop by my desk and interrupt me to say the same thing.
There's a low signal/noise ratio and the noise is overhead.
And that's exactly the situation people customised Gmail to do (using filters etc), and what Inbox is designed to do from the start.
The problem is that "noise" is very, very context sensitive. 30 messages of commit-spam, 10 comments on a bug and a 60 message long thread in an internal mailing list suddenly become very, very relevant when that bug lands on your desk.
Large scale engineering optimizes the teams output, not that of individual contributors.
A key component of team output is communication.
Email is a key communication mechanism, especially within Google where (to a large extent) their development workflow is built around it.