Lets not go to far with exclusives. That puts people AWAY from Xbox, so that's not going to happen...
Hence why I said when Halo 5 comes out... If Halo's main draw is its online and when Halo 5 comes out, most of the community moves on, then there is no reason for MS to not port MCC to PC... Cause a) MCC sales will drop on x1 b) MCC will be significantly less active and c) Most people will be playing Halo 5 instead so if they port it to windows so they will not only get a sales increase on MCC, but it will also show some good faith to PC gamers and if they do it right, it might be a reason to visit Windows Store for once