I'm sure the battery will hold fine. My family has a passed down iPhone 3g and the battery does not last very long but its been used a lot and is almost 6 years old. I'm sure the mi3 battery will last many years until its old and slow and you can get a much better one. And I'm sure he battery will be easily Changeable if needed.
The same what happens with the iPhone, HTC One, Sony Z, and every other fu** new Smartphone that comes out. It gets old and if you want to replace it you have to pay.