If that were their route they'd have to drop their investment in QnX, try to adapt all BB software to run under the latest incarnation of Windows Phone rather than to just run Windows Phone on it. BB users are quite particular about their handsets & associated software, much more so than Nokia users ever were.
Of course this could be totally wrong but I think the challenges in moving BB to Windows Phone are far more substantial than to get Nokia to convert even if BB is a smaller player.