The point is that here are already hundreds of guns available. And there are only a handful of primaries that people use frequently now. If we keep all of these weapons viable, they will be preferred above everything else that comes in the expansion, nullifying the effective new weapons. Plain and simple. If people can use Fatebringer for ten years, they will. If they can't, they have to move on and find new things to use which is better and breeds new diversity which destiny desperately needs.