Non-replaceable battery has got to be one of the most profitable examples of planned obsolescence. Your average consumer is going to notice that their phone is lagging more frequently and attribute that to their phone being old… when in reality it’s just that their battery has degraded [1]. So, what could’ve been a <$100 replacement becomes a $1000 upgrade.
[1] https://support.apple.com/en-us/HT210557?cid=iOS_SettingsUI_...