They already got the models to work (we won't see them in game otherwise), the only things missing are their face in selection screen and dialogues.
Communications and events. I don't know if they plan on three idol communications, or three idol events, but if they do, having those four will increase the amount of stuff they have to do considerably. It's been gone over before, the amount of combinations of idol groups there are.
But, when you get down to it, if all those dialogs are like:
Point: We need the leader to say X, your leader is Haruka and she delivers it like this.
Point: 2nd idol responds to X with Y, your 2nd idol is Mami and she delivers it like this.
Then in my opinion, it's not much more expensive to include 4 more characters (and the dialog would seem pretty... hollow). If however, the dialogs, and the direction of the dialogs, are all unique (in that, say, Haruka and Chihaya both respond with different ideas, and dialogs are tailor-made to suit them), then I can kinda see exactly how much content you need.
"Oh well, they're still going to buy it anyway, because it's Idolmaster!! MUWAHAHAHAHAHA!!!"
*waits for cash to come in*
I'm more thinking that, with corporate politics, there's a lot of finger pointing and a lot of I-told-you-so going on. I mean, first you have to tell this to the people making the game, which also consists of fans. I mean, you saw the reaction that certain fans had, but you're telling these guys to make the exact thing that they railed against. And for those people making the game that aren't fans, you still have a PR crisis on your hands.