On a lighter note, the protocol might be proprietary but the bridge still seems to be fully open source : github.com/ProtonMail/proton-bridge
I don’t think think Proton shows bad will on this one. The only alternative I can think of (as a non expert) would be IMAP + GPG encrypted emails but very few desktop clients support GPG, which would make them less accessible 🤷♂️ Having their own protocol also probably makes it much much easier for them to iterate on it, opening up usually makes think much robust but also slower.