The webmail provider released a dedicated browser that can only open the webmail and called it a "desktop" app.
Additionally, they don't support IMAP. There's an app to run on your computer that becomes a bridge. The proprietary protocol is translated to IMAP. You can't use your favorite client if your operating system can't run that bridge and you're not a premium user because for "reasons" only premium users can run that local bridge
They don't support IMAP because they want emails to remain end-to-end encrypted, and IMAP doesn't have any way of doing that. The gateway decrypts the emails locally, then serves them as plain text.
We need something better than IMAP, that's designed for modern use cases. Something that's not stateful... Maybe a web service or something like that. JMAP seems promising but barely any providers have implemented it.
Still, if an user prefers the convenience of using any client instead of e2e, could enable it in a setting. Maybe the user subscribed because they liked the interface and the overall features of the plan, and not because of the encrypted email solution and just wants to add the account on the mobile client instead of a dedicated app
Being closed like this IMHO is just to increase user retention
If thex subscribed because of the interface (ehich is certainly plausible), what would they need IMAP support for? Also, if you really want IMAP, xou can have it, you just need their (open source) Proton Bridge for it (thats a sofrware) so that ut retains all features. But then I would need my own email client.
On mobile you're forced to use their "open source" app that is only available on the closed source app stores and not on fdroid because it uses Google push services
Even so, your statement that it is only available on closed-source app stores is wrong. And it doesn't even matter that it's not provided by "My First F-Droid Repo Demo" (yes, that'd the name of the official repo). Many open source apps are on IzzyOnDroid, including Jerboa, what do you use to write on Lemmy?
Either way, your original comment is completely wrong and it doesnt help that it's "only" available in the most popular extra repo.
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.