In an interview with Eurogamer, Microsoft Xbox head Phil Spencer confirms what many suspected – Rise of the Tomb Raider will only be exclusive to Xbox One for a "duration" of time.
When asked to clarify the terms of the exclusivity deal Microsoft signed with publisher Square Enix, he said, "I have Tomb Raider shipping next holiday exclusively on Xbox. It is Xbox 360 and Xbox One. I'm not trying to fake anybody out in terms of where this thing is. What they do with the franchise in the long run is not mine. I don't control it. So all I can talk about is the deal I have. I don't know where else Tomb Raider goes."
Later he added, "Obviously the deal does have a duration. I didn't buy the IP in perpetuity."
So, while the game will definitely launch in fall 2015 exclusively on Xbox One, there's a good chance that it will eventually be released on other platforms, including PlayStation 4.
This is what I suspected all along; given the economics of the making triple-A games now, it's hard to forgo the possible sales of an entire platform like PlayStation 4 – especially when its the leading new-gen console right now. However, I'm sure the deal Microsoft and Square Enix signed will be beneficial to both parties. Microsoft gets a flagship exclusive title for next holiday season and Square Enix gets (presumably) a good deal of cash.