I am not sure if that is a big issue for MS since they will making either way. It will probably end up on PC making MS and Remedy even more cash :). It will remain exclusive to MS though, that much is certain.
Same thing with Titanfall, no one takes the word exclusivity seriously with Microsoft. Aside from that though, I based on Remedy's history I think they'll get Microsoft to let them put it on PC, just like a year or two later. Saying something is exclusive to MS isn't saying much. As long as you have a PC you're set, really.