The Zig maintainers clearly think that keeping up with the undocumented native API is less headache than using the documented but notoriously inelegant win32 API.

This might very well be a good idea. Microsoft is not going to change a vital piece of their OS just on a whim. I would wager that even if they wanted to, they would not be able to do so that easily. A large organization maintaining a large software with a billion users just does not move that fast.