They do "seasons" where you buy an expansion every year, and the expansion includes all of the (admittedly pretty significant) updates and added content for that year. It's not the best model, especially since the content of the expansion is delivered over a year's time, but it's their alternative to a subscription and how they continue to fund the game. I personally don't mind it, mostly because you don't need to buy the expansion if you don't want to, and there usually isn't a strong incentive to do so until they've added more that you want.