My 4s with 7.1.2 is still running good. Its battery just replaced last year (not in apple store nor apple service center). My wife's iphone 6 is still going strong with IOS 8.2 (jailbroken) and very responsive as well. I even still have 3gs used by my daughter, running IOS 6 and still functional as a phone and basic smartphone which is still sufficient for her.
Even though it is recommended by apple and many security expert to update the OS to the latest due to security risk, I dont really buy in the idea. Possibly its security breach can happen but at very minimal rate (I havent heard it in practical daily life); also my phone do not contain extremely high confidential data either.
[doublepost=1479111594][/doublepost]Yes correct. In Indonesia (where I lived) there are few small workshops that can replaced soaked iphone battery with very reasonable price (about $20-$30). They replaced with used battery but with just few cycles (mine is about 90 cycles) and they will not upgrade the OS