That's always a concern with any game with a long alpha period. However, the other way to look at it is that every additional feature from now until it's released fully is a potential moment to reinvigorate players. Imagine all the possible features and then imagine which of those would make you personally sit up and think, "gonna have to give this another go". Multiply that by the bazillion people already playing it or have yet to play it.
Another concern I have with this game is that everyone will get burned out on it before it's even released.