> For a counter example of how this "un-scalable" development model can actually work just fine--regardless of OS--look no further than the DRI2 and KMS changes coordinated across X.org, other user space and the Linux kernel, all at the same time with many hands involved in the work and with very little user-visible disruption.
Your counter-example actually illustrates my point. It is relatively easy to coordinate changes between the kernel and a single userspace project such as X.org. Once you try to make kernel changes that impact many userspace programs, it becomes very difficult to coordinate the necessary changes. The story of what happened with the Linux TTY changes is concrete evidence of the drawbacks of the split kernel v.s. userspace development model.