Because it will still be effective for a good portion of TTK (up to lvl 34 content) and still viable in crucible. The ramifications will be a fresh set of builds for an almost completely redesigned game and Bungie can finally start correcting the mistake of making weapons that are more effective than any other weapon could be to the point that there is no other logical choice. My biggest concern is whether the story will finally be engaging, whether there will be characters I can finally care about not whether my old weapons will hold up to new ones.