I think that you might be underestimating the appeal that games with female characters (customizable or not) can have. It isn’t a coincidence that a lot of titles that have a female character option are extremely popular in a lot of gaming circles, on websites like tumblr for example you see a lot of active, mostly female, fans for the franchises that offer this option. And the female audience is only growing, they already have the guys.

They also already have a title that had a female assassin as protagonist, so going “they can’t make 1 out of the 4 co-op characters female because it isn’t profitable” clashes with the entire existence of AC: Liberation. And there it was a full commitment. Here it’s only one out of four.

Tbh, my main annoyance is that it’s pretty damn lazy. Four different characters that they’re so far using for marketing, cinematics and co-op could’ve been given much more variation and are a golden opportunity to appeal to a broader range of players. But they just seem like the same standard gaming character basics sheet while plenty of companies have already proven that varied characters in good games work perfectly fine. "They can’t make the main characters for the walking dead game a black man or a girl because it wouldn’t have profited." This argument is only used when a game is lacking something that for some reason is outrageous to mention, but when it’s done with success nobody brings it up.

Also Ubisoft shits out AC titles like they have anal leakage. They have resources and money. It’s a cashcow.