Why would anyone be interested in efforts on a platform with a closed-source backend and that is not developer focused?

Because most people don't care about those particular things. Almost all the world uses completely proprietary tools (Gmail) that also violate your privacy.

Not to mention, entirely unnecessary why you should have to use a bridge gateway in the first place with IMAPS & PGP/GPG, CalDav & CardDav. Like I said, Proton is engaged in some questionable practices.

It's not unnecessary, it's the result of a technical choice. A winning technical choice actually. PGP has a negligible user-base, while Proton has already 100 million accounts. I would be surprised if there were 10 million people actually using PGP. They sacrificed the flexibility and composability of tools (which results almost always in complexity) and made an opinionated solution that works well enough for the mainstream population, who has no interest in picking their tools and simply expects a Gmail-like experience.

And if you really have stringent requirements, they anyway provided the bridge, so that you can have that flexibility if it's really important for you.

IMAPS & PGP/GPG, CalDav & CardDav

• IMAPs is just IMAP on TLS, so it does not have anything to do with e2ee in this context.
• PGP/GPG is what they use. They just made a tool that is opinionated and just works, rather than one which is more flexible but also more complex. Good choice? Bad choice? It's a choice.
• *DAV clients expect cleartext data on the server. If you encrypt the data, you need to build all this logic into the clients, and you are not following the standard anymore, which means you will anyway be bound to your client only (and those which implement compatibility). Proton decided that they want to implement e2ee calendar, and they decided to roll their own thing. It's up to everyone to decide whether e2ee is a more important feature than interoperability with other tools. I don't care about interoperability, for example, and I'd take e2ee over that.