That's an intriguing idea - it would imply that the phone has a valuable lifetime of 3-4 years or more, which is not the current sales model! ALL of the phone manufacturers want us to believe that we really should be upgrading every year, or at least every second year. The AUP shows that pretty clearly - you are entitled to upgrade your phone after 12 months, and it's financed over 24 months. I think those two periods say a lot about what Apple thinks and expects. I do think we're going to see consumers choosing longer upgrade intervals (longer than 12-24 months), but that will cause a lot of painful adjustment for the phone makers, I think.
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Re: building in the US, it's not even clear that the US has a labor force that is able to do what's necessary to assemble things like iPhones, at least not at a price anyone could afford (as you say). When Apple tried to build computers in the US (in the last century), it was apparently a disaster, and things are probably less favorable today:
Apple Computers Used to Be Built in the U.S. It Was a Mess. - The ...https://www.nytimes.com/2018/12/15/.../apple-california-manufacturing-history.html