Re: Borrowed technology from the Dreamliner batteries??
I haven't actually looked inside an iPhone of any generation, but only the simplest of things don't need at least a couple of screws. There are many possible variables, but inside Apple, or in a company like ours, you've got several people constantly working to maximize efficiencies of every part and every process. The calculations are exceedingly complex, take weeks to work out, and in the end you've saved .00031 cents per unit, but because the screw manufacturer has price breaks at 500k screws you end up saving 1.2 cents, per phone, and reducing logistics and labor costs because without the additional screw an entire bin of 60,000 screws is consumed between each days parts cycles. Before you always had a 1/3 full bin at each parts cycle and were having to pay for someone to not only bring in full bins and take out 1/3 full bins to the parts dept where you were paying another person to operate a machine you had custom built to open the partial bins and mix them with other 1/3 full bins to create a full bin. Now you're talking real money being saved. You've just taken nine full time people out of every line producing that (widget) (say 20 lines for 180 total employees no longer needed there) and all for one tiny screw per unit. If you want to go even further, because you're completely using an entire bin of screws you can now order 1,100 bins less and eliminate 350 truck deliveries and cut back/reassign loading bay staff and the extra space lets you reorganize the warehouse and reduce pick times .0002%. This is all because of a single screw remember.
That's a summarized, fictional, but valid, outline. The actual processes are much more complex. Those processes are repeated every time something changes (logistics company instituting or eliminating a fuel surcharge for example).
The point is that each category of component/part has a generic value associated with it for rough calculations of its total, actual, cost. Screws and wonky connectors are just absolutely awful to mess with. Adding or eliminating one screw will have enormous financial implications and if there's a screw in there its existence has been throughly examined zillions of times, just like every tiny part in a modern piece of tech kit.
Money in manufacturing is made in fractions of a penny repeated millions and millions of times.