> None of these reason should justify buying a new phone, but in practice if replacing the part is not relatively cheap and straightforward, this is what happens.

The reason that is what happens is that you can't upgrade the old phone. But you have to upgrade the phone anyway, so you take advantage of the only way to do that, buying a new phone.

I wasn't upset when I smashed my phone screen recently, because that phone already needed to be replaced for other reasons.