"Privacy by default" is a good motto when the mechanisms used to ensure privacy actually work most of the time. I'm sure that between ProtonMail users, it does indeed work most (if not all) of the time.
But once you leave the ProtonMail ecosystem, I expect you'll find that enabling PGP automatically doesn't work most or all of the time. It likely only works occasionally or even rarely.
> You can't change the world by copying someone else :)
Please take this as gently as I intend it: ProtonMail is unlikely to change the world on user privacy, at least when it comes to email. Larger providers are too entrenched, and most people won't care enough about privacy to change their email address (and very few people have a custom email domain to move between providers).
At any rate, it seems like you're copying Facebook: "move fast and break things". Not great when we're talking about messaging. You're breaking email for some people, and some of those people are your users. OP may not be one of your users, but OP's mom is, and you have broken her ability to send her son email with this misguided policy.