This is a silly thing to take issue with. I use a password manager. When I need a new password I allow the manager to generate one for me. Is the password inherently insecure or bad because it was generated by "a company" and not myself? Proton generates your key for you, just like a password manager does, and they've integrated that functionality into their service for ease of use, and probably ease of administration as well. There is no way someone can screw it up and not be able to read their emails if Proton handles it.

Encrypting email is extremely niche in the first place, the fact that Proton can enable it quickly and seamlessly for users with no prior knowledge on how this all works is a good thing imo. Everyone with just enough knowledge to think they know better seems to get annoyed by this type of thing and starts spreading ridiculous FUD even while Proton is enabling encrypted email for millions of people who otherwise would be using Google Mail. Don't get so caught up in the details that you miss the big picture of what Proton is actually providing.