I don't think assembly-line complexity was Apple's main reason. I'll bet it was more of a marketing thing. They know the initial run of iPhones will be snapped up by the zealots, and so they didn't need to offer a cheaper model right out of the gate.

After all the zealots have paid top dollar for the first iPhone, and the initial buzz has died down, then they'll offer cheaper models. At that point, they'll get all the customers they didn't get the first time 'round because of the price. See, those pricepoint customers will still be there later on. Whereas, with the zealots, they only had that one chance to milk them for every penny.

That's my theory, anyway.