Its already flatined. The only ones left are the hard core players that support Bungie's failures, players who play because of their online friendships and players who now play every now and then. Anthem was supposed to be the game that brought the final nail to Destiny, and that too failed as a game. It failed worse than Destiny has done over its lifespan.
One day there will be a game that will deliver and will not push its player base through the crap Bungie and so many other developers have done. When that happens it will be the end of these studios. D3 will not be that game.