Well, I'm not sure that the XR will get 3-4 more years of iOS updates, but that doesn't mean it won't work. With that usage - yes, I think that the battery will last 3-4 years easily. Also, my wife and I just did the same for her Mom this year. Not her first smartphone, but she had a 5C that was soon going to lose the ability to make calls (as it doesn't support VoLTE and Verizon is soon dropping all 3G/CDMA cells), so we gave her my wife's old iPhone 8 with a new battery. (She thinks of the iPhone as her "carphone". She uses an iPad for almost everything.)