>There is only a "depreciation" model if you don't unlock your bootloader

That's fair. I also think most kids won't know how to unlock a bootloader (and if Google has its way, that won't be an option anyway).

> a seven year support cycle makes it very easy to obtain cheap, old, fully supported phones, if that's your jam.

Is that iphone support? I think only the biggest flagships get 7 years of official support (and can maybe run 2-3 more years before app version itself out, without external intervention). From what I saw, most budget phones are lucky to get 5 years of support, and will break down well before than anyways.