Halo 2 map packs were sold in stores. Map packs have traditionally undersold and split the playerbase late in the game's life cycle, so their price justifies thier existence. This content is integral to the continuation of the main narrative in a game that has a dirth of content to begin with. There's no one out there who didn't feel like they get into the next Halo because they didn't drop $10 on a map pack from the last one, but we may see people less likely to purchase Destiny 2 based on the fact they couldn't justify paying $40, the total cost of the next two "expansions" and 2/3 of the initial investment, on a few levels that heavily rely on assets created for the vanilla game.