A fresh new game generates more income than an 2-3 years old game, whose veteran players have already reached the endgame content, grinded quite a lot of gears/levels and began benefiting from the initial investment (time + money) they have done.
An old game doesn't attract many new players due to the huge gaps between newbies and veterans, which cannot be compensated with money, so paying players always prefer new game/new server in order to gain more advantages.
A conscientious publisher would run the game longer, even the game becomes less profitable, because if people spent more than 100$ in a game, then they usually intend to play the game more than 2 years.
A cunt publisher would shut down 2-3 years old games and add new games continuously within short periods.