I should probably get fitted for a tinfoil hat because my immediate reaction was, "Oh shit! They must've developed something now that tracks you better and is less obvious."
My educated guess would be that given the proliferation of services offered by Google, it indeed has many more sources of information on each individual, and having email as one of them perhaps proves less beneficial today as it did when Gmail was first introduced. Now there is the Android OS which gives information about apps you use, locations, activities, and online searches, map queries, bookmarks, sentiments via…
Google's new problem is "How do we get access to the contents of these new communication channels so we can continue to read peoples' conversations?". Those new channels are walled gardens and you can't just create a new email client and say "Switch to GInsta for your Instagrams! We give you 2GB of storage!" Unlike the switch to Gmail, the switch to GInsta would represent a hard barrier to the "true" Instagram network.
Google is becoming the new MS; losing relevance with the younger crowd who are all about "social" platforms, and becoming more and more an enterprise sales model. What happens when Facebook goes after web search?