I have other email addresses, and some of them have public keys, but the metadata is not private. Protonmail to Protonmail is better, not perfect, but better.
Without strong passwords, secure and private applications are not secure or private.
When you communicate with me, if your end is not secure or private, then you are throwing my security and privacy into the garbage. I don’t have a way to know that you are being reckless or careless, and I don’t have a way to ensure you are protecting my security or privacy.