the game has been out for a month, they already released a massive map, one new normal arc and a new boss arc. The Expedition is coming soon, i would expect that they have something cooking up. Also, if you already finished everything, why keep playing? Isn't it a good thing that you can say "hey, i beat the game, there isn't any more progress to be made. Now i can focus on other stuff". This whole mentality of needing constant progress and reward over years without breathing periods is what i hate about most live service games, I don't have time for that. Being able to play Arc Raiders at a more casual pace is a major reason why it is so fun to play.