Typically, replacement parts are only manufactured during the lifetime of the device. Because of that, I don’t think that the replacement batteries would contribute significantly to waste.
People will also perhaps then buy replacement batteries when handing down/selling devices, where they would have just have managed with degraded performance in the past.
What’s the problem with this?
It’s fine, as long as you enable 2FA for Google and make sure to maintain access to the account you’re secure. It won’t have all the fancy features that some of the other apps have, but if it works for you then it’s good.