Depends on who you ask - if the NT kernel were to be made open-source it wouldn't impact Microsoft's sales of Windows because the kernel is useles without the userland - and the real value of Windows derives from the userland Win32 API.
If they mean the entire OS - kernel, userland, shell, the bundled programs that come with it - no, that won't happen, not least because Microsoft doesn't own the rights to all of that code: any third-party licensor could put a stop to that overnight.
If they mean the entire OS - kernel, userland, shell, the bundled programs that come with it - no, that won't happen, not least because Microsoft doesn't own the rights to all of that code: any third-party licensor could put a stop to that overnight.