You're missing the point. A phone is often given to family or friends when someone upgrades, so it continues being used after many years. Especially in the current commoditized market where phone upgrade cycles get extended and people hold on to their devices for longer (also considering how much they cost nowadays), 3 years of OS updates won't cut it anymore.
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Once the phone falls outside of the initial purchaser's needs (Average iPhone user, for example, retains phone for 2-4 years) and is moved on to family/friends or sold, I don't think the manufacturers are remotely interested or concerned in providing that long term support for that device or it's new adopters. They provide the relevant time support periods deliberately aimed at the initial owner. Clearly manufacturers would rather friends/family buy a new handset as opposed to picking up the older model. Whilst the manufacturers all talk a good game in environmentally friendly phones and sustainability ultimately, as we all know, it's just about money. No money, no company.