I agree with OmarL. Just because some site is fading it doesn't get vintage - especially not with RC.SE, as, despite a athe generic liberal handling, our definitions are made for easy exclusion, not automatic inclusion. Then again, we do accept rather fringe areas as well - various arcade questions might give an example.
Then again, browsing the questions at WindowsPhone.SE makes me think that these support orientates questions might draw a quite negative response from our core audience.
Bottom line: I do not think that merging the sites will be beneficial for either.