Time is always a killer literally for every game to gradually lose players. Without doubt, every game would finally comes to the end where there are no more people playing the game. In order to keep as many players as possible while time is passing, game developers are supposed to think of tons of ideas basically for replay-ability. As time goes on, all kinds of games (no matter how good the games are) would lose players fast when players get bored. Even though you are a royal blizzard fan, you would get bored on only playing WoW. Blizzard kind of adjust that well because there is a Overwatch that probably would catch back some previous royal players to play WoW again.
This game got a ton of players when it was released due to it new AR technology that people have never seen before. However, novelty doesn't necessarily mean it would never make players get bored. The reasons why Pokemon Go is losing players so fast are mostly because of its repetitiveness. Basically, Pokemon Go is only doing one thing that is catch the Pokemon. As novelty wears off, players would not think it is interesting any more, let alone there is only one mechanic to play with.