Robb won some battles, began losing the war, made a promise, broke it, made some stupid mistakes and boom.
He certainly wasn't the last main character to be punished for his mistakes, especially in A Dance with Dragons.
Don't get me wrong, I liked Robb. I hated the Red Wedding. I don't think I was supposed to like it. But, I'd rather have a world and story that keeps me on the edge of my seat because nobody is safe rather than having everything spoon-fed on a formulaic level here.
On the other hand, if Martin proceeds to systematically kill off every single character by the end of the series, that is just as bad as letting everyone live happily ever after chasing rainbows and kittens.
I know people who stopped reading after the Red Wedding. Omg Robb! Omg! Omg!
Again, Robb was never A HERO in the story (if we are speaking about main protagonists). He isn't supposed to have a splendid heroic arc here. Bran, on the other hand? Absolutely. Arya? Yes. Jon Snow, yes. Dany, yes. Tyrion? Remains to be seen. Same with Jaime.
I think Martin did a great job at showing us who the movers/shakers were from the start. Ed was not one of them. Robb wasn't one or he might have gotten his own POV. Heck, the Hound seems to have more heroic potential than Robb, imho.
We'll just have to see what happens. If they go and off Tyrion, I'll probably change my tune. But, that hasn't happened. Yet.
In the end, we all react to material in a certain way. I can appreciate why someone would really not enjoy the series after the Red Wedding and want to just hang it up. But many of us don't feel that way. Maybe those of us who are okay with it are just big ole masochists. (grin)