Everyone is blaming the loss on the fact that it's a mobile game, but Pokemon GO's decline is mostly due to all of the bugs and glitches present with the game (most notably, the tracker issues). Niantic really needs to get it together before the decline continues
Did you see what the latest patch fixes? Now you can ask your class trainer for an idea about what a pokemons stats might be... (or use an IV calculator on them, far more effective) and now your Pokemon will correctly faint when returning from Gym protecting rather than remaining at 1hp.
I mean... Niantic are really on the ball with combating the issues the real players are having with this game... I was always so annoyed that my Pokemon didn't require a revive, and now they do, I can see millions of these players rushing back now that this has been fixed!!!