At that point my phone is usually cracked and worth upgrading, but with each phone I go through I try to take better care of it. But so far I've never liked a phone so much that it was worth replacing the battery. But I have bought the exact same model of phone 2-3 times as replacements (esp when I broke one by dropping it)

I don't know. I usually buy used pixel devices, but that's a good point. If you are trying to plan the replacement costs for an iPhone and you can repair the battery for $30-80, that's a steal.