> Phones from 10 years ago don't work any more because wireless companies are dropping 3G.
This is because of a phase mismatch - a phone bought 10 years ago would have been in the middle of the 3G lifetime. 3G started getting adopted in 2002-ish and had ~200M users in 2007 according to WP[1]. That's between a 13 and 20 year window for usage - which overlapped with 4G during the tail decade (or so), so, in fact, there's a good chance that a 10-year-old phone would work today because it could have been released with a 4G radio (and, if it was built to be a 10-year phone, it absolutely would have had one).
> If the goal is to make parts replaceable to reduce waste, what is the difference between keeping a phone for five years and throwing it away to buy a new one, then keeping that for five years, vs replacing all the parts in "one" phone over a ten year period? Two complete sets of parts are used.
Different parts have different Mean Times Between Failure (MTBF), so you don't end up replacing all parts at equal rates. CPUs tend to last far longer than batteries and screens - having a repairable phone means that you can replace the battery several times over a 10-year lifespan, whereas you're almost never going to replace the CPU.
That is, buying two five-year phones results in you paying for two CPUs, two batteries, two baseband modems, and two of everything, regardless of which component in the first actually failed. Buying one ten-year repairable phone results in you paying for one CPU, one baseband modem, one display driver, ..., ...and two batteries. Far less e-waste. Every time a non-repairable phone breaks, you have to throw the whole thing out. Most times a repairable phone breaks, you can fix what's broken.
Additionally, he MTBF of a composite, non-repairable device is lower than the MTBF of its least reliable component - meaning that (theoretically - I'm ignoring the durability costs of building dis-integrated devices) it's more expensive to build a single, non-repairable device that lasts five years than to build a device composed of components that will individually last five years each.
> What is the difference between ten people owning ten 10 year phones, and ten people each handing down a phone that one person bought new?
As it stands, most phones sold nowadays won't last 10 years, so the latter scenario doesn't really happen. Some manufacturers (Apple) make relatively reliable devices, but those come with severe freedom and privacy restrictions (the argument is that reparability should be a right, not a feature of a particular company), and their devices are infamously non-repairable.
[1] https://en.wikipedia.org/wiki/3G#Adoption