At my studio we maintain a native Linux version with a custom game engine, and it indeed takes a lot of time. I don't consider Proton a viable option as we lost the ability to integrate with Linux-specific stuff such as Wayland APIs or better input, but I can definitely see the appeal of switching to Proton... if your team uses Windows. If you have some developers on Linux, you naturally get a Linux build (if using cross platform APIs ofc) and it's actually faster to cross-compile a Windows build every once in a while (skip the slow ntfs I/O) and ship that. But it requires getting more of the team on Linux :)