While Windows itself may be proprietary, it mostly runs on fairly open hardware. Open in the sense that the manufacturer doesn't impose artificial limitations on what software is allowed to run on it, and also provides very thorough documentation for it, giving no advantage to any party in regards to information.
Also Windows itself is open in the sense that it doesn't restrict any software from running on it. You can write user-mode applications and run them on any architecturally compatible Windows system. You can write drivers to run kernel mode code on any Windows system (you can disable signature verification).
In the case of Windows, it being proprietary doesn't actually limit what anyone else can do on it nor is it artificially limited to only run on approved hardware. In many ways, it is open.
Yes, that was exactly my point at the end. You need an open system in the sense that no one controls access to it, but it doesn't have to be open in the sense of open source.
Also Windows itself is open in the sense that it doesn't restrict any software from running on it. You can write user-mode applications and run them on any architecturally compatible Windows system. You can write drivers to run kernel mode code on any Windows system (you can disable signature verification).
In the case of Windows, it being proprietary doesn't actually limit what anyone else can do on it nor is it artificially limited to only run on approved hardware. In many ways, it is open.