For this to make any sense, Microsoft would have to admit to itself they have a problem. So far there's no sign of that at all, in fact the endless WP astroturfing about security and business use strongly suggests MS believe WP8 is the business phone Aaron is looking for. Or will be when they finish the missing bits and smooth off the rough edges.
What would buying RIM have got them? The BBM system? How long do you think MS could resist just folding it into the wider Microsoft network, tying it to your Microsoft account and inevitably pissing away most of what made BBM so attractive to business. Even if they didn't screw up the service, their reputation would damage the product.
RIM didn't get success by building great smartphones (everyone I know with one hates their device), or building a great smartphone OS (they didn't). It was the service running behind it all people bought and Microsoft buying that service would devalue it enormously and instantly.