I have a 2012 iPhone5 with iOS 6.1.3 still on it. I never upgraded it. When iOS 7 came out, all the news reports said it killed the battery. Same with iOS 8, 9, and finally iOS X.
Yes, I assume that eventually, iOS point release 7.x.x fixed the battery issue but I don't care to keep visiting news websites to figure out which exact version is finally "safe" to upgrade. I don't want to be a slave to the housekeeping of my phone.
Others say the news reports are alarmist -- all one has to do after a new iOS release is to dig into the settings and disable all the new features/polling/etc that eats up the battery. But it's the same situation -- I don't want to put the effort into learning "what's new" that I have to disable.
On the other hand, I'll quickly upgrade major releases of MS Visual Studio from VS2015 to VS2017, or upgrade Webstorm from 2016 to 2017.
I was puzzled over my contradictory approaches to updating software. I think I figured it out: my phone feels like an "appliance" to me instead of a computer and I don't want to mess it up. Another reason is that Apple won't let me pick an exact version to upgrade. I can't pick iOS 7.1.2 -- I'm forced to upgrade to the buggy iOS X or nothing at all.
As a result, I have willingly performed an "IE6" legacy lobotomy on my iPhone.
(As trivia, I also notice that iPhone5s on ebay that still have iOS 6 sell for a slightly higher premium.)