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I get what you're saying, but it doesn't help practically with the idea what I don't want to buy a new phone all the time.

If I wasn't so wed to Android the right thing to do would be to buy whatever iPhone was new at the time and ride it down until they stopped providing updates. Especially because (anecdotally, of course) iPhone battery life seems to last much better over time than Android, so not only will I continue to receive updates for longer, but it will actually be a usable phone for longer.