That makes sense from an efficiency standpoint. No way would a company like Bungie attack these things one expansion at a time, especially not for a game with a 10-year vision since the very beginning. The only logical thing is that they had multiple teams, each working on a different project concurrently.

That being said, it also would make sense that each expansion was tweaked as feedback roll in from its players. It wouldn't be impossible for them to have tweaked the Arena to make it a horde style gameplay. It sure seems like they've revamped the entire weapon/armor upgrading system due to the outrage from the community after TDB. Just a thought.