Daryl dies when TPTB decide that it's time. No way to predict when that will be. The show is mainly about Rick anyway, and Rick's character has gotten a lot more interesting this season. Andrew Lincoln's acting is some of the best yet. The evolution of his character is what it's all about.
Seems like a lot of people are more focused on what's going to happen to the show's supporting cast these days.
I find it a little odd but that's just me.
Norman Reedus is a part of the Main cast. Supporting would be along the lines of Carol, Hershel, Beth, Milton, Sasha
But i agree with you though, there's really no way to predict when Daryl will die like we can the other characters.