You are all crazy saying the RW wasn't as big a deal as Tywin.
If Robb had been able to ally again with the Freys, the North would have been in no danger of losing anytime soon. They could have marched up and easily got rid of the Ironborn. At that point he probably allies with Stannis and now the forces in Kings Landing have a serious problem. They surely don't want to try and fight a war in the north as winter is coming down.
On the other hand, Tywin dying has almost no immediate effect. Cersei and the Tyrell's take over. They are still in charge of King's Landing, their armies aren't in trouble, etc.