Because people <i>do</i> replace their phone, and also uninstall&#x2F;reinstall the app or wipe app data etc.<p>You could argue (and I have done) that a key change event should be surfaced in the UI as &quot;Your contact has switched to a new $MODEL_NAME phone&quot; so if it occurs in the middle of a conversation or you happen to know they really haven&#x27;t upgraded, it can lead people to tap it and learn more. An alert like &quot;Contact key has changed&quot; would be meaningless. But it&#x27;s also the case that notifications that seem useless to people rapidly get phased out, and unfortunately any notification that boils down to &quot;you MIGHT be being attacked by your own government but PROBABLY your friend just dropped their phone&quot; is a non starter.