I apologize, but I don't have the time to put together a considered response right now, though I wish I did. Maybe this evening.
I do think it's interesting the amount of anger I encounter whenever I even remotely question one of the Singularitarians' babies. There seems to be more ... emotional attachment ... than is healthy for skeptical inquiry.
You would encounter less frustration if you engaged others' arguments rather than dismissing them out of hand. That doesn't mean you have to argue with every crank on the internet, but there's not much point in writing a comment which declares an argument wrong without giving an explanation for why.
I'd very much like to hear what you have to say, because even when I've discussed this with academics who work in nanotech (though I've never spoken to anyone working directly on replicators), I've never heard a better argument than "it's really, really hard".