Destiny is an Online Game, meaning no Offline support, that means the game requires servers to run which need to be funded and maintained. Maintenance on Hardware also costs money and requires staff to manage it, Bungie does not charge us a Monthly fee like most Online Games/MMOs which run from servers, instead we are left wanting more and buy expansions for £20 every 4+ Months, this balances for the most part about a fee that would be a monthly subscription. Look at ESO for Example, you had to buy the game to even be able to download it, and you had to pay a monthly fee on top of it. Companies do need to make money also, I believe the cut content and DLC prices was their reasoning for not implementing a monthly fee into the game.