That would rely upon sales of the Wii U or the XB1 to completely stagnate, which within the first two years of release for a new console would signal the death nell of a platform (like the Dreamcast).

So as much as I favor the PS brand as my go to choice for gaming, I sincerely hope not.

The only other way for this to happen is as Mummelmann previously stated, would be for SCE to sell more hardware units (20m) than they are currently capable of physically manufacturing.

Making a prediction like that (20m sales to hit that 50% marketshare), would be like Nintendo fans making Wii sales predictions that outstripped Nintendo's manufacturing capacity.