vv221: It’s not about stubbornness. Unless I’m misremembering you are a developer yourself, so you know that you can’t realistically change your whole software build toolchain just to include an alternative renderer, especially with a codebase as impressive as the WINE one.
Not change, but they could at least make an effort to accommodate it. I think it's doable. But they didn't want to. Understandable on one hand (i.e. their idea is that if you want something better - build it yourself and add those dlls which anyone who cares to have good performance on Linux does). But on the other hand, they could make it more seamless.
May be another factor is project governance. dxvk and vkd3d-proton are independent of Wine and can make their own decisions and move fast. May be they didn't want to be governed by whoever makes final decisions in Wine project.
Language choice in my opinion is not a blocker for project inclusion though, and having strict requirement for C only is what I would consider stubbornness. But I don't think it's the real reason here anyway as above. More likely a combination of macOS problem and managing the project.