As an employee in this type of survey, I tend not to ask for things I assume would never happen. So if I disliked the open-plan (which I don't), I don't think I would ask for a private office. I would think that the open-plan is a given and not something that could change in the whole company because as soon as you give a private office to one of your developers, you have to do it for everyone who wants one which is not practical without moving.