This guide is nice for people that are motivated, but a large chunk of users (probably the majority, but I have nothing to back that up except a guess) will only encrypt their emails when it's as frictionless as sending an unencrypted email.

Encryption tends to be successful when it's baked in to the system (e.g., WhatsApp theoretically), a preset default (e.g., Android 5.x+ and iOS), or requires absolutely minimal effort (e.g., the HTTP -> HTTPS "switchover"). This has been a known issue for a pretty long time [1, 2].

[1] http://www.gaudior.net/alma/johnny.pdf

[2] https://cups.cs.cmu.edu/soups/2006/posters/sheng-poster_abst...

Once you've set up Enigmail with your PGP key, it is as frictionless as sending unencrypted mail.

I use Enigmail to send encrypted email on a daily basis. It couldn't be much easier than it is already.