> This will be done by notifications and direct messaging. Kids these days don't bother with email.
Pg is not talking about personal emails for messaging. Personal messaging is mostly "read, reply, and forget", and you are right that this is now better served by instant messaging protocols (modulo the fact that there is no universal IM protocol).
He is describing work emails, as a "todo list" as he puts it. You get such emails from your colleagues or business partners. They generate work for you and you cannot just reply and forget about them. You need to work a long time (either doing research or the actual work), and get to that email thread later. It is a "shared" todo list, and you send messages to synchronize the state.
Google's Inbox is built exactly for this purpose, although I believe pg wants something more revolutionary.