I usually keep a phone four to six years, have had iphones for the last few decades, and have not had any issues with batteries. The battery life and speed in charging does improves with each newer generation phone of course (one of many numerous nice features when I do do an upgrade). But as you can see I don't upgrade all that often.Do Iphone batteries often fail or not last very long? I have only had android phones and never had any battery issues. I agree was someone mentioned above, changing a battery on any sort of tiny electronic equipment likely takes special tools and skills to do correctly. You could also damage the device if you do something wrong. Seems like one of those things I would only have the experts do.
My dad who is 88 and had the battery issue has a seven year old phone and preferred not to get a new phone. His preference at 88 was that he didn't want to have changes or have to learn to use any new features. Also he uses it primarily for phone calls and asking Siri an occasional question about that weather or the stock market (That's about it), so we were not concerned about him not being able to get the latest updates. He keeps it plugged in almost all the time which I'm sure is not good for the battery in that older generation iphone, and I'm sure contributed to the battery issues. Dad is happy and totally delighted with his new battery.