Like many other titles in the video game industry games depreciate in price. Developers do their best to capitalize on the initial hype. After that production decreases. GTA4 while having a multiplayer component is primarily a campaign game. Once the campaign is completed replay value falls. These games get traded in to stores for newer titles. Stores like Gamestop (GME) then can sell the used title to value shoppers cheaper. this increases the depreciation and reduces the demand for new unused titles.
Other than GTA4 episodic content there is no stable revenue stream for TTWO to fall back on. They will have to invest heavily into newer titles. That will erode profit margins going forward.