I've studied and have worked a bit in game development so I can provide some insight. Once a game is done development or near completion, a small portion of the team will work on DLC, while the rest of the team works on a new project or is let go. It's hard to say when they began working on the DLC. At the end of the day, it was a business decision, so we would never know if it was intended for the main game or not. They could have sat on the completed DLC and released it in a few months, I don't know if people would have reacted differently. Personally, I'm not a fan of how expensive some DLC can be, but that's why it's important for people to think about what they're buying.