Do you think GRRM’s claim “Anyone can die, even the heroes” is true? After all, Ned was only the main character of the first book, so he’s obviously not the hero of the story. Robb and Cat died, but they’re not important characters and Robb wasn’t even a POV character. Jon died, but he’s sure as hell going to be resurrected. So, if neither Jon, Tyrion or Daenerys die before Book 7, does it mean GRRM was bluffing?

That’s a rather presentist way of looking at it, no? When the first book was all there was, Ned was the main character, and he died. That hadn’t really happened before in genre fiction. 

Robb and Cat were hugely important characters, the driving force of House Stark’s war effort, and we can tell from the impact that their death had on entire kingdoms. 

Add to that the death of Khal Drogo right when we thought he was about to invade the Seven Kingdoms, the death of Renly when it looked like the war was his to lose, the death of Tywin at the height of his power, or the death of Oberyn on the brink of victory…the list goes on and on.