I said at the most basic level, as in, hey, here's this extremely oversimplified yet fundamental issue to collaboration you get in private offices, which is that it is harder to communicate. Regardless of your thoughts or beliefs, it's just plain easier to ask someone on your team a question face to face rather than needing to open up an IM/chat/mail/hangouts/whateverthefuckitis and do it through there. It's not only easier, but it's far more fluid and will result in both quicker answers and more discussion since you don't need to open up a program, draft up a message, then wait for the other person to type up their response. It all happens in real time, quickly. And in an open office, you get this for free, you don't even need to move.
Meanwhile, you have not provided a single source to back up the pretty bold claim that private offices don't impede collaboration and instead decide to straw man that argument. I've explained the incredibly obvious and basic impediment to communication that a wall can bring, please refute that claim and provide proof that a private office does not impede collaboration.
My last workplace had private individual offices for developers. My office was big enough to have two people working together in it full time, and big enough to have 4 people in it comfortably for 2-3 hours.
With that in mind let's talk about communication. There are a few different types of frequent office communication. Let's compare an open office with private offices for each of them.
1) a coworker is blocked and can either spend 10-15 minutes figuring out the issue on their own or can ask you to get an immediate answer.
In the open office he will usually ask you and pull you out of focus. This has a large negative impact on productivity when it happens throughout the day. In a private office scenario he works it out himself. His problem solving skills increase, and you stay productive. Big win for private offices.
2) a coworker is blocked and it will take her a day or two to work it out herself or she can ask and you can tell her the solution immediately.
In the open office she will ask immediately and you save two days at the cost of impacting you for 30 mins. Winning. In a private office she will struggle for 15-30 mins, make no progress, and then walk over to interrupt you. The open office scores a small win here.
3) a coworker is not blocked and you know the solution.
In an open office they will interrupt you. In a private office workplace they will send an email/IM and you can respond asynchronously without impacting focus. Big win for private office.
4) a coworker needs to work closely with you for an extended period (30 mins+).
In an open office you either hunt for a meeting room, or because that's usually a pain, you talk at your desks. This either holds up shared resources (meeting room) or disturbs everyone (talking at desk) so you keep the conversation/collaboration short. If you have a private office you can speak without worry leading to more collaboration. Another big win for private offices.
There are a few other common scenarios in the workplace but I think I have made my point. 1, 3, and 4 are more common and are big wins for better communication in private offices and only come at the small cost mentioned in 2.
