Whats your incentive to keep playing after you´ve beaten all the nightlords?

I´ve killed 3 so far and naturally I´m only going into playlists to hunt for the remaining ones. Is there any benefit to kill the other nightlords over and over again? Isn´t that a waste of time? (well, apart from simply enjoying the game, of course.) Don´t you think the overall interest in the game will decline once most of the playerbase managed to beat the nightlords?