I kinda struggle to believe it's that difficult. I mean, Tresorit has a pretty good and functional Linux client. What have they done which makes it sustainable for them?
Filen.io also has a pure sync-client, which is distributed as an AppImage. This also works, but the FUSE integration Tresorit provides is quite awesome and performing quite decently.
I would actually recommend Proton to start the development on an older Linux distro. Like RHEL/Alma/Rocky 9 or Debian 11 (which is EOL, though) and make it run there. Moving from that distro to newer distros will then go smother and you'll get other distros supported quicker.
The mistake too many Linux efforts does is to take the "latest and greatest" distro version - often coupled with what a single Linux developer considers the "most used distro" and then hits lots of challenging when needing to support older distros. That's going to be painful.
@protonprivacy Please take note and forward to Andy and other managers.
@mmmm @codeinabox Sure it can.
Microsoft can distribute their Windows Desktop Environment for Linux in a repository they control. And that repo can even contain just the binaries, with their own proprietary licence. These packages can further have dependencies to other open source packages.
If Microsoft ends up with their own Linux distribution or just mirrors an existing distro (like what Alma/Rocky does with RHEL, or Ubuntu with Debian) ... That depends on how much control they want over the Linux distro base OS.
But they certainly have the possibility to add a Windows experience as an alternative to GNOME, KDE, Cinnamon, XFCE, etc, etc, building on top of a shared base OS layer. As well as providing WINE like layers to make existing Windows programs run in that environment.