Really seems unlikely though - would surely hurt XB1 sales wouldn't it?
Of course it would. I can hear MS talking to the investors that love the Xbox brand so much: "Yeah, we're gonna put Halo on PC so it brings in some extra money... even if it stops consumers from purchasing an Xbox One. But we're continuing with Xbox anyway, so you can just suck a fat dick."
They'll gladly leave your money on the table and take the money from a console sale, a game sale and a XBL subscription.
Especially since it would probably be a purchase when it's $5 on Steam.