I don't know. I am no software engineer, the only thing I can do is to hear what experts say. And by his record, Hubbard is an expert.Why launchd, or why Darwin?
He did a great talk and mentioned problems which they had to get Mac OSX on mobile devices etc. and how they solved them. It could very well be that these problems can be solved without a different init system. But the experts seem to use launchd-type of init systems.
I red today that Ubuntu already 2007 wanted to use launchd for their init system but because of Apples open source licence they rejected it.
Edit: It was 2006 and not 2007 . I looked it up again ...
Edit2: To be precise: In the Wikipedia article about launchd, they said that Ubuntu considered using launchd but rejected it because of licensing. This reads a little weaker than what I formulated above ...