Its maybe a bit premature to be assuming that this is going to be a survival mode or that it will all take place in one location, they havent really said anything to that effect yet. All they've said is that its a 'brand new three player cooperative multiplayer arena'. I don't know if we should assume that they literally mean an arena (after all a brand new arena would suggest we already have one, which we don't), which is why people are assuming it will be a survival mode. Given their language in other posts, and the context I'd say their using the word arena to refer to a new game mode rather than an actual colliseum style play area. They'd probably describe the crucible and strike playlists as combatative multiplayer and cooperative multiplayer arenas respectively too. Also if you check the reef out now they've added space for four locations so hopefully their planning something a bit more ambitious than just a survival mode.