"tell all developers that they are free to look at the sources"
I see where you are coming from but I think that would kill Windows as a platform.
Developers would look at the current source code and write apps that depend on behaviour that is currently true but which is merely an accident of the current implementation. Since Windows apps are typically sold as closed source and typically not updated for free by vendors to track OS changes, the result would be that each new version of Windows would break about half the software that you've paid for, with fixes only available if you pay the vendor again.
As readers of Raymond Chen's blog will know, this already happens to a debilitating extent. That's surprising because the only way to create such dependencies right now is to reverse engineer Windows. Apparently some programmers are smart enough to walk over assembly listings and reverse engineer how Windows currently works but not smart enough to realise how fragile this is. Worse, many of these programmers do this even when there is a documented alternative.