For this one, you also need to explain why we can't reverse-engineer it from the human brain.
It was designed by evolution. Say what you will about the blind idiot god, but it's really good at obfuscation. We could copy a human brain, and maybe even make some minor improvements, but there is no way we could ever hope to understand it.
I'm not saying we'll take the genome and read it to figure out how the brain does what it does, I'm saying that we run a brain simulation and do science (experiments) on it and study how it works, similarly how we study how DNA transcription or ATP production or muscle contraction or a neuron's ion pumps or the Krebs cycle or honeybee communication or hormone release or cell division or the immune system or chick begging or the heart's pacemaker work. There are a lot of things evolution hasn't obfuscated so much that we haven't been able to figure out what they're doing. Of course there's also a lot of things we don't understand yet, but I don't see how that leads to the conclusion that evolution is generally obfuscatory.
If Strong AI turns out to not be possible, what are our best expectations today as to why?
I'm thinking of trying myself at writing a sci-fi story, do you think exploring this idea has positive utility? I'm not sure myself: it looks like the idea that intelligence explosion is a possibility could use more public exposure, as it is.
I wanted to include a popular meme image macro here, but decided against it. I can't help it: every time I think "what if", I think of this guy.