It's actually very possible to ignore the Material widget set for desktop apps. And in fact, we do have Cupertino widgets that support macOS, although they're principally designed with iOS in mind. That said, it's true that short-term goal isn't to reimplement the full system desktop control set on every platform we target.

Material isn't just intended for web apps, of course -- it's a flexible design system that can be highly customized and branded. And we're seeing that many new desktop apps adopt their own look and feel. For example, the Adobe Creative Cloud suite doesn't look like a 'stock' Windows app. Even Microsoft Office or Edge use very few system controls: they're almost entirely custom drawn to the needs of the app.