My bet: End-to-End encryption, not even automatic but through the use of a plugin, will only appeal to the niche of people who are concerned about that and may not be using GMail. For the most part of its user base, nothing will change.
Things will start to move for the majority when end-to-end encryption can be done directly in the protocol, by default, i.e anything but email.
Things will start to move for the majority when end-to-end encryption can be done directly in the protocol, by default, i.e anything but email.