Imagine if Bungie open-sourced Destiny and Destiny 2.

Sure, you don't have to buy the game anymore to get the software, but they also don't have to let you onto their immersive and highly moderated and orchestrated servers. They can offer their own signed builds that you need to access the official universe.

So you could still break the game apart and build new content for fun, but the main game could still stand on its own and still be easy to sell to people because you're not selling the world, you're selling the experience.