Apart from the discussion surrounding the crypto side of all this, one thing that ProtonMail has really succeeded in doing is making encrypted mail finally an accessible thing to a non-technically inclined user.

I've spent well over a year trying to get my team to properly gpg encrypt sensitive files only to constantly find them sending plain unencrypted files over slack, dropbox and even simply gmail.

At least now, finally, they've found ProtonMail easy to use and are choosing to send files this way.

I think the key, unfortunate reality is that until the end user doesn't have to think about encryption, they aren't going to bother using it. Bringing usability to encryption is another huge part of the problem that needs to be considered as it is basically a guarantee that the typical user will take the path of least resistance.