Slight problem there. If MS does this Nokia will drop Windows Phone like a hot stone, so in the gap between Nokia abandoning it because they won't want to compete directly with MS, and MS actually getting their own BB engineered phone out (likely to be several years), WP would have zero market presence. Surface has already royally pissed off MS laptop and tablet OEMs. This move would critically sour their relationship with the only viable WP hardware vendor.
I understand where you're coming from, and Microsoft may well come to decide that their best strategy is in-house hardware, but there's a major downside to such a move.