Because would you really want to use the same guns for the next ten years? Also gjallarhorn is too powerful and so will always be the main exotic for end game content (unless they make something equally or more OP which I doubt they will). The new exotics will go unused because everyone would stick with gjallarhorn. The game should not have to be based around gjallarhorn (eg: bosses having more help to combat gjallarhorn's high DPS) which leaves people without it in a bad situation. What don't you agree with?