They're not going to be useless, are weapons we love will play a huge role throughout the START of the taken king. Bungie will be slowly fading out the year one legendaries and introduce the year two legendaries that will be stronger. For example if reforging sticks around for weapons we might be able to get a hand cannon very similar to fatebringer but a stronger and better weapon all together. Exotics of course will stick with us throughout the ten years of Destiny.