There is no list of changes they could make to get a hundred thousand people to come back - changes that, according to the wants of forums posters, essentially entail undoing a huge chunk of the game - a feat that is in many respects probably technically unfeasible, and largely counter to their commitment to their "vision." It makes more sense that they try to keep the ones they still have. That means the populations will continue to be shit and we have to wait for Halo 5 to see if they learned anything.