Just remember that the 6 was the oldest phone the current OS supports. I've had no problem with iOS 10, but by next October, Apple will likely introduce another one and who knows if the 6 will be supported?
That said, if the phone works and the apps you use work, should you care? Up until about a year ago, I had an iPhone 4, whose OS support ran out with iOS 7.1.2. Apps ran fine and I was good with that, but the upgrade to my 6 was because the battery was fading and I also liked the larger screen - not because of it's 'better' OS. An OS which has not, for all practical purposes, changed all that much for years, beyond its hardware demands. See also OSX / MacOS in that regard - but I digress...