Uh!
I think open source is great. And I suspect the widespread belief that manufacturer support for software guarantees its security may be largely wishful thinking. But really, don't you folks think that open sourcing Windows 7 might lead to hackers worldwide picking over the code looking for, and exploiting, Windows 7 security flaws for fun and profit? And how many of those currently unidentified Windows 7 security flaws might still be present in Windows 10?
Open sourcing Windows 7 really might turn out to have unintended consequences.
Not that it's likely to happen anyway.