Who are claiming its peaked then? R*?
Like everyone else has said, its their own fault. You carnt claim its peaked due to sales figures from a spin of game, released only on one console. I dont own any GTA's past GTA4 for the PS3, because I simply carnt . Ive never considered buying an x-box 360 to get the exclusives either. All this move did was alienate R*'s dedicated Playstation customers. Whether Microsoft saw more consoles being sold because of this I dont know... I dont know anybody who has bought a 360 just to get the later GTA's.
Its definatley not peaked. GTA4 was just the start of a new era of GTA gaming, there should be many more, improved GTA games on the PS3/XBOX, until the next gen console is released in 5 years time and we all start again.