Apple plans in obsolescence, but battery life hasn't been the major issue. For example, I think one of the bigger issues for the 5 and 5s are their 1 GB RAM. Unfortunately, for the 6 and 6 plus owners, they also have that 1 GB RAM limitation.
Apple's iOS manages RAM well, but with each iOS update, machines with less RAM suffer more. 128 MB iOS devices became obsolete really, really fast. So did 256 MB iOS devices. 512 MB iOS devices lasted much longer, but now are irritating to use as well. 1 GB is next. In fact, I sometimes already find 1 GB iOS devices irritating to use, whereas 2 GB works much better.
That's why I tell people if you're going to buy an iOS device in 2015, try to get one with 2 GB RAM if you plan on holding onto it for a while. For an iPhone, that means waiting until the fall.