Even today, you still need an external program to your email app to enable encrypted messages with in the email app. With that, the number of encrypted emails will be extremely low relatively. Even looking at Tor, the amount of users is still much lower than the internet in general. To the point, I doubt a very high amount of traffic is "regular" traffic to raise the noise floor.

Huh, Thunderbird ships with a PGP implementation by default? And it's actually very easy to use.