They have to monetise it, which is totally fine, as long as it's not some silly pay-to-win service. Path of Exile's cash shop, and similarly how other games work like Last Epoch, where you can buy epic bank space and cosmetics is the best approach. They'll not be able to run the game purely on game and expansion sales alone; that's where they failed from Diablo 3 after they took the RMAH servers down and they ended up with no money being generated by the game to fund additional development.