I think most mobas that have been around for a while always decline in player-base, and that is usually how it is for most games. A problem that has been killing most mobas are smurfing or account boosting which kills new players experiences. Higher Ranked players in mobas get bored in their ranks, so they create new accounts that are lower ranked. This makes it hard for new players to join and enjoy the game when they are getting mopped when they first start playing. So I don't think the game itself is dying, I think it's hard for new people to enjoy the game, which leads to a static player count.