A potential change in Valve's culture/management aside, "let valve do the work" is a feature, not a bug. Studio spends all their budget targeting one platform (which still has ~90+% of the PC gaming market), and get Linux support for free.

Windows' monopoly on game dev isn't just market share either, since game dev isn't just code. You still need Photoshop, Maya, etc. and in smaller studies there's typically a crossover where some devs are doing art as well. Visual Studio's C++ debugger is still one of the best, and the tooling elsewhere hasn't caught up yet (compared to DX + PIX).

Then you also have to solve distribution and handling the fragmented display & audio stack. It's gotten a lot better, but its still a factor.

I'm fine with most of the work going into Wine/Proton. A stable ABI for Linux is a boon, if it happens to be Win32 then so be it.