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The article does answer that.
The article points out a few shortcomings in launchd, but admits it does most of what he wants just fine. His only concrete objections are (1) not all existing daemons would be compatible with launchd, and (2) launchd is not flexible enough to do everything he can conceive.
I believe it would be wiser to use a working, release-quality codebase than to write something from scratch. If more backwards compatibility is needed, it would be much simpler to add this functionality to launchd - there is nothing in its architecture to prevent this. And to answer his second objection, sure, launchd is not perfect, but it's much, much better than init/cron/inetd/xinetd/whatever.
Plus, if Linux adopted launchd, daemons could be written to be compatible with both Linux and OS X, and developers from both communities could contribute to the further development of launchd. Both OSs would benefit from sharing an open standard.
Well talk in cheap and the blog does talks about the advantage of being platform specific. If you want to talk to the author and get to know more details, the blog post is open for comments :-)