To be fair, putting out significant additional content ~3 months after release is a fairly tall order if you think about it, especially if the main game has come in "hot".
If you think about it, the team has most likely been crunching hard to get the thing out, and by the time the final is accepted they'll be ready to take a well-earned break. That'll certainly be how management will have presented the situation to them.
But, if the reality is more crunch to fix significant bugs, plus start work on the additional content that needs to be finished in another few weeks... that's just going to be demoralizaing. Especially if its a project that's had less then total buy-in creatively throughout on their part, and now it basically needs to be redeemed in the eyes of the audience... Just saying, if you found yourself in that position how happy would you be in your work?
Not to say that users shouldn't feel disappointed, angry, or misled. My point is simply that this isn't the work of malfunctioning robots, its made by humans who are going to react to the situation they find themselves in, and that reaction is going to impact their performance and productivity. Which in turn is going to have further effects on the project.