They will get their chance. Episode VII is the only one that will focus on the classic characters. I want to point out why the prequel sucked (other than the scripts) is that there were no holdovers from the original. Sure, it's the same characters, but younger. It's still not the same as having the actors that brought life to those characters in the first place and that's why having them get their send off is the right way to go. There has to be a 'passing of the torch' to the kids. They can't just show up and carry the film, especially if the original actors are in VII and used only in cameos. It would feel very odd and actually draw attention to itself.