Ask Microsoft if they will place the XP code base into the public domain and let others support it. I can guess their answer.
Yeah, they'll tell you not to be so
unrealistic.
If you want open source (Linux) or mostly open source (OS X), then go that way.
Microsoft never have been and never will be an open source developer, nor are many other major software developers (e.g Adobe, other than their open source file formats e.g. PDF).
Put yourself into their shoes, they are private for-profit company, perfectly entitled to make their own decisions about their own intellectual property and when they cease support for an obsolete product.