I've replaced batteries very successfully on Nexus and Pixel phones. They're usually $30-$40 or so... I have gotten crappy batteries from Amazon but I've never had a problem with ifixit supplies. And if you're not into doing it yourself, you can just take them to Asurion or whatever. I'd be totally shocked if battery replacement costs more than $100... so nowhere near the $400-$700 original price of the phone. If you're comparing the battery to the resale value of the phone, maybe you have a point. But you're not buying a used phone with a fresh battery, either. Batteries generally cause enough issues that I plan on replacing them every three years.