There's just so much abuse going on, and when you are privacy first you limit your ability to fight abuse which makes things much worse. :(
I created EteSync[1] a contacts, calendars and tasks sync solution that's e2ee and privacy respecting but people kept on asking us to integrate with one of the above email solutions. Though they are all closed and don't interoperate. :(
I'm still moving my stuff to mailbox.org now though, I'm getting increasingly annoyed at having to use proton's mail app in addition to my main mail app.
Proton doesn't interact at all with the rest of the ecosystem. I'm not a Proton customer myself, I just hear it from our many customers that are frustrated by their closeness.