Ego Depletion Phenomenon
This is a theory that suggests that we have a limited amount of self-control and that this pool is reduced every time we exert it. Basically, things that we are not highly motivated to do become very much harder if we have already exerted self-control doing other things. The physical activity element of Pokémon Go whilst being an initial draw card for the game, becomes another hurdle to overcome once ego-depletion kicks exceeds the intrinsic benefits.
Ego-depletion and the decline of users in games are invariants. There are very few ways to avoid the dynamics of people giving up doing something which has no sunk cost and no life-altering benefits. Pokémon has started off from a higher base of users than any other game, but its decline will follow the same course, and over the same relative path as other games.