That is Tywin Lannister's plan, yes, but remember during the scene with Walder Frey and Roose Bolton (he was the one who killed Catelyn), Walder and Roose were talking about the recent killings. Walder Frey said that Edmure Tully (Catelyn Stark's brother) "spent his wedding night in a dungeon cell"; Edmure Tully is still alive!
Here are some things to consider:
1) Walder Frey is a man without honor and not a whole lot of nobility. He is known around the Seven Kingdoms as an old man who likes to marry young girls and hold on to grudges. He may think he's done the Lannisters' a great service by killing Robb Stark and his bannermen, but 2) this actually put the Lannisters' in a very difficult position. They sent out ravens with the message that Robb Starks' banner men turned on him and killed him, but Sandor Clegane (The Dog) is still out there (and Arya Stark), and they both know the truth, along with Edmure Tully. 3) Those 2 boys that Theon Greyjoy "killed" were not Bran and Rickon Stark, so he (Theon), along with 4) Samwell Tarly know that the Stark boys are still alive (Those boys that Theon had killed were a local farmer's adopted boys; the farmer sold them to Theon. Theon had to make it look as if he were hardcore and in order to get their loyalty, he had Bran and Rickon "slaughtered").
This season of Game of Thrones did not cover all of Book 3, so there is still more to come. Tywin Lannister thinks that all of the Stark males are dead; Bran and Rickon are not and don't forget about Jon Snow! You can't count out Edmure Tully, The Dog, or Tyrion Lannister, who (after learning from Jaime of something that his father did to him) sees his family from a much different point of view. Also, Jaime is back in King's Landing; don't be too surprised if he has learned what true honor is from being around Brienne of Tarth.
When the series does return in Spring 2014, it will most definitely pick up where Book 3 left off and go into Book 4.
