This game must be expensive for them server-side though. Network integration is everywhere in this game, not just matchmaking. Your inventory, stats, reputation, everything is constantly synchronized with their servers. No information is stored locally. Surely they can't afford to provide this sort of infrastructure indefinitely...
As the population eventually starts to drop over the years, it'll become cheaper and cheaper to run the game.
In previous Halo releases, they kept optimizing and increasing the datarate of the next game so much that the -entirety- of the previous Halo's online system ran as a small instance on the current Halo's backbone. Halo 2 lived on in Halo 3's server room. Halo 3 lived on as a part of Reach's rack units. And each time the datarate kept going up. Halo 2 could take up to an hour to show recorded games. Halo 3 could update every 30 minutes with interactive stuff. Reach could update every minute.
Destiny is capable of updating every
second. This is what even lets you do stuff like manage your in-game inventory from the web. They needed to do it in the first place to get the MMO stuff to work right, but it's been amazing to see a direct evolution of Bungie's servers like that.
As a side note, Bungie also just took over hosting Halo 1 PC's online after the Gamespy shutdown. That game is 12 years old, to give you an idea of how long they'd like to run things.
I can imagine by the point Destiny 3 or 4, or whatever Bungie does after Destiny have microsecond update rates and site backends that running Destiny 1 will be like "man, remember when we thought this was hard?" to them.