SWMBO and I are iPhone users, used iPhone users, I with a 5S and she with a 5. The 5S is still on the original battery, which will be 4yrs old in March, and while it does get me through a typical day, it does not get me through a travel or conference day of heavier usage. SWMBO's 5 has had the battery replaced, thanks to ifixit for parts, tools, and instructions.
Both phones meet our needs, though the 5 stopped getting updates in July, and SWMBO is limited be the 16gb. Offering $29 battery upgrades (ifixit charges $20 for the battery alone) will likely be enough to sway us into upgrading to 6S by the end of 2018. Holding off that long because we're not heavy users - an out of date flagship phone does us just fine, I'm cheap, plus we hoping to build a new house in 2018 (which will be far from cheap).
I might have had a point when I started this post, but I'm not sure what it was now...