That is how mmos have always worked. From wow to other ones, once a new expansion is out, all of those weapons that you loved so much, become weaker than that. They will ALWAYS allow you to upgrade your exotics through Xur or by randomly finding the item again, but as far as legendary raid weapons go, those will become useless when newer content is out. They are aware that we are upset, but there isn't much that they want to do about it.