I actually mention that this thread isn't about budget/time concerns in the OP specifically because adding this in would obviously require more of both and Ubisoft has already shown that, at this point in production, they believe that can't add in playable female avatars.
If the game is releasing in October, it is 100% guaranteed that it is content complete and likely was months ago. Cert, QA, and stability testing is all that is likely going on now.
No one outside of production knows specifics related to budget, scheduling, Etc. However, anyone who reads/understands how the co-op is projected to work can come up with conceptual solutions, which is what this thread is about. If it's unreasonable to discuss anything about a game without being intimately familiar with its development, discussion on this board would cease to exist.
Even if I was concern trolling, it wouldn't change whether someone can provide a conceptually sound solution or comprehensive reasoning why this isn't possible. No one's provided any of the latter so far, I'm still open to more of both.
Literally nothing is impossible when you exist in a non-reality without budget, time, or workload constraints.
There is nothing preventing AC from having combat as good as Dark Souls, attention to graphical detail besting GTAV, the ability to enter every building, unique dialogue and animation sets for every NPC, a fully rendered country side, the predation system from Far Cry 3, a branching storyline with dialogue choices like The Walking Dead.
None of these things are technically infeasible. Some of them would take an astronomical budget (combinatorially or alone) and some would just take a different focus.
Saying that there isn't budget or time or focus isn't a reason you want to hear doesn't invalidate those things as reasons. I guarantee that when the designers were sitting around coming up with what they wanted for co-op, they probably thought they were being awesome when they found a way for everyone playing to maintain "an emotional connection with our lead hero, Arno"..."never breaking immersion, integrating the narrative through clever illusion." It just happened to be this game where everyone finally lost their shit at the lack of playable female characters. It's been a long time coming, and it's a fight worth having, but pillorying this game for it (which probably had its narrative design nailed down years ago) is kinda baffling.
And then we come to you. You keep wanting to hear a reason why it isn't happening and "you haven't been convinced yet"...yet the reasons are plain as day. So A) what are you looking for? and B) why are you looking for it?
You aren't asking the question; you're saying every reason isn't good enough and no one is going to convince you. Ubisoft screwed up one more time in not allowing female playable characters in co-op. They didn't plan for it, it was cut due to budget/time, or they are monsters.
Seeing as they're one of the only publishers to have a black female lead (non-sexualised), a native American lead, and an anti-Christian lead (who would also be considered of Middle Eastern descent), it seems unlikely that it was done because of sociopathy.
Game development is trade-offs. Features are added and taken away based on budget and projections. They didn't think that the audience would have a problem with everyone being Arno and they were wrong. The voices were/are loud (and for good reason) and future projections will likely take this into account.