There's nothing stopping Bungie from building the foundation to allow cross-play. Sony is just being a butt. I think it would be hilarious however if Bungie actually did implement cross-play and cross-platform fireteams, Sony says "No, we don't want that.", and then Bungie just locks Playstation users out of that feature while letting XBox and PC players use it. Keeping in mind, this would not negatively impact PS players compared to the way things are now, they just wouldn't get access to this "new" feature. But it would sure as hell light the fire underneath Sony even more so than it is now. If players knew that they wouldn't be able to have that feature if they bought the game on PS, then they might not buy it on PS, and that is the opposite of what Sony really wants.