That
is the point. Valve is maintaining the patches; Apple has to do
nothing besides support compliant Vulkan 1.2+ drivers (which their community already does).
Apple doesn't need to implement ntsync to support Proton or DXVK, either. macOS' synchronization primitives do the job just as well as esync or fsync does.
> why are we all going through so much effort to save game devs from writing good OS-independent arch-independent code?
Because Apple doesn't support OS-independent APIs. They also don't support old runtimes, so users will inevitably have to run the Windows version once the native code breaks.
Valve ported several of their games natively to macOS, and almost every single one of them is broken on modern Macs. Valve didn't change the game tech, it's all runtime breakage.