They won't or can't IMO. I was watching reruns of it the other day and realized that when a lot of the dust of the books settles the show could really take a hit in viewership. Ned,Robert,Renly,Drogo,Joffery,Robb,Tywin,Catelyn,Jeor Mormont...all dead and Tyrion exiled,Loras Tyrell severely injured, Jorah banished from the Khalisi,Theon gone....that's more or less most of the main characters from the first couple of seasons. They're either going to have to drag stuff out to extend these characters on the television or count on the Briene, Jon Snow, Cersi and Jamie characters (and Actors) to carry most of the water. I can't remember a show that would have such turnover in characters in such a short time.
I also hope that they don't foreshadow Robb's demise too much as that could be an "all-time" television shock moment....even more so than Neds death.