Yeah, I know. Impossible. But just go with me for a minute.
Metal Gear Solid 4: Guns of the Patriots was exclusive to PlayStation, and Metal Gear Solid: Peace Walker – what Kojima and Co. routinely called "Metal Gear Solid 5" – was also exclusive to a PlayStation platform (the PSP). It's true that the upcoming Metal Gear Rising: Revengeance will be multiplatform, but we know Kojima has since distanced himself from that project; Platinum Games is in control of completing it now. And the "Solid" was removed so we all understand that it's a spin-off and not an official entry in the franchise.
Now, we know Kojima is working on a number of things; likely, one of them is MGS5, which we haven't heard about just yet. Given the time frame and with the expectations that Sony will launch the PS4 later next year, it wouldn't be a stretch to see the next MGS on the new PlayStation iteration. Furthermore, Kojima has expressed his dislike for Microsoft in the past; some years back, when asked if MGS would ever come to the Xbox, he responded with, "Not if I have anything to say about it." And if you hadn't noticed, no official MGS has come – or is scheduled to come – to the Xbox platform. Another important fact to acknowledge.
Therefore, why can't we assume MGS5 will be exclusive to the PS4? The timing and situation seem to be about right, even if it's becoming clearer and clearer that exclusive titles are extraordinarily risky these days. It just seems that after Revengeance , everyone expects all future MGS titles to be multiplatform…and I'm really not buying that.