Seems like a bad business model. From previous articles, it is clear that some development time during the original release of the game, went into the expansion. It also seems that they are clearly planning to monetize the game by constantly releasing paid content, which will segment the community, and if there is any PVP, then also cause some pay to win elements since the expansion will have newer and better gear.
The original game was heavily criticized due to its lack of content and much of the time being spent on repetitive grinding and dealing with bullet sponges that follow fixed action patters.
This will likely anger many people who has hoped to buy a game that will improve over time rather than having to constantly buy new updates to the game every few months. (imagine paying for early access to a beta on steam and having the developer charge for updates).