Metro Apps are not based on Win32 and you can have a GUI without installing Win32 at all as is the case with Windows 8 on ARM. This significantly reduces your

Metro Apps are not based on Win32 and you can have a GUI without installing Win32 at all as is the case with Windows 8 on ARM. This significantly reduces your threat model at the cost of backward compatibility. They developed the Metro Start screen specifically for Windows 8 ARM but shipped it across all OSes using that Kernel for consistency. I think it was starting with Windows 8, all GUI server tools were just a UI running powershell underneath. So you could scrap Win32 and still have full control of the system.