If you pair it with modern antialiased/high-DPI typography, it still looks clean and legible.
Some later busier looks -- MacOS X Aqua or NextStep, for example, have a lot of complexity (shadows, reflections, more complex widget designs) which needs to be manually remastered for updates for higher DPI or changes in overall dimensions. But classic sharp bevels can be pretty much scaled up and down infinitely and remain crisp and on-brand.
Maybe that's the thing: the "brand" they're on-brand with is the system's brand. The widgets don't blend into any specific website or company branding diagram, because they were designed to fit with the Windows style guide and goal for software to look and act consistently from one program to another.