Earlier quoted context omitted.
Proton developer here, working for CodeWeavers. Everything we develop goes preferentially in Wine, and is kept in Proton as a "hack" only when a solution clean enough for Wine is unfeasible or too complicated. Proton is periodically rebased onto Wine, so everything in Proton and not in Wine is additional rebasing work, which we'd like to avoid as much as possible.
Thanks for all the great work. I hadn't realised Proton was developed by CodeWeavers. Out of curiosity, what is the relationship between Proton and your own product Crossover?
In practice Proton and CrossOver tends to have rather different sets of needs. Proton only supports Linux and is only focused on gaming. CrossOver exists for a handful of platforms, most notably macOS (which has a number of intricacies: missing 32 bit libraries, Apple Silicon, missing Vulkan drivers which requires MoltenVK, but MoltenVK itself cannot really offer all the features standard Vulkan drivers have on Linux), and is also used for non-gaming applications. CrossOver also lives in the form of specialized branches we develop for specific applications, on request of their developers. So while of course we share everything that can be shared, in my experience this tends to be not that much (except what goes upstream, of course). In pratice CrossOver and Proton live in different repositories, have independent build systems and independent release schedules.