For me it's been less the battery and eventually the screen itself. As the phone gets older, getting replacement screens or glass ends up more and more difficult and expensive to the point where it can cost almost 1/3 to 1/2 the cost of a completely new phone to fix my old one. At that point it's a harder decision to make to fix the old one when a single drop after that could mean that I've now spent as much to get a new phone (2x replacements) to fix the old one.