I recall Ingo arguing against a plugin architecture for the CPU scheduler, and saying something along the lines of "The disk scheduler people probably regret making theirs pluggable". I don't remember his exact words and nor can I find where he said it, so this is a para-phrase.
Maybe they do. But this demonstrates the sort of innovation that can happen once you expose a stable API. At the very least I imagine much of the infighting we see around the CPU scheduler would have gone away if Ingo had seen fit to loosen his control of the CPU scheduler by doing the same thing.