I suppose I disagree that a distinct corporate body is required for autonomy, but I would definitely agree that without such the risk of the autonomy disappearing in the future is ever-present. I think Red Hat has tons of autonomy now, but at some point IBM could start changing that. It's also possible they have, but having worked at Red Hat and knowing many of the people there, I would expect a lot of screaming, whistleblowing, and resigning should such a thing ever happen. Red Hat is a remarkably "speak your mind" culture, and people do even though it sometimes starts shit storms (such as when people call out the CEO for something publicly on the company-wide mailing list).