OP is a generally good post. Two minor comments.
Open plan has the advantages of being familiar, allowing more people to fit into the same space, and being more egalitarian. I.e., since offices are viewed as status symbols, providing them to developers and not to other groups could have a significant negative impact on morale among non-developers.
Open-plan seems egalitarian, but it's not. It's the opposite. If you're a manager and your boss is on another floor, you don't have to worry about what's on your screen, how often you go to the bathroom, and people coming up behind you (only interns will do that) when you're obviously not receptive. It's subordinates who end up feeling like caged animals.
When you work in an open-plan office, power relationships are shoved in your face 8 hours per day. If you're the rare sadistic middle manager who enjoys watching people squirm, you can do "the Boss Walk" all day and enjoy it. Everyone else (even decent managers) is miserable. If I were a manager, I wouldn't want this: I don't want people to feel threatened and anxious (or a need to change what's on their screen) every time I walk by their desk on the way to take a piss.
That the engineers at your company are more productive, or more in tune with the needs of the business because of the constant natural interaction with their coworkers.
I worked for an R&D think-tank during a summer internship. Everyone had an office, but at 3:00 there was "tea" and people got together for snacks and board games for an hour. They naturally ended up having this kind of conversation.
There are better ways to achieve this. Open-plan offices may populate the social graph, but they flood it with antagonistic edges. What the fuck good is that? None.