instead of just using an open protocol like XMPP they opted for their closed thing in order to lock people into their apps
That’s just not true, you’re severely misinformed on this.
Proton took the established practice of PGP encrypted email and put it in a nice package. That’s why you can add public keys and just message somebody that’s using Thunderbird.
There is no “open protocol for end to end encrypted email”, XMPP is not applicable here. There’s no “IMAP for PGP” there’s just IMAP, so they made a bridge so you can use IMAP even if your mail client doesn’t support PGP.
Could they have made an IMAP server that returns the PGP emails and requires your mail client to handle the decryption? Yes. However, that goes against a major selling point of the product which is that it manages all that encryption for you (like a password manager). Nobody in their right mind would use that.
This isn’t some matter of privacy coolaid and fanboyism; they did the open interoperable thing. You can even (as an example use case) if you’re a new customer that was doing PGP email on your own, upload your own existing PGP key, and use that with Proton if you don’t want to change the PGP public key people use to send you email.