When you absolutely should not upgrade your iPhone

In the past few months, Apple has released frequent updates to iOS 11. Should you upgrade? For most people, it's a personal preference, but there are some people who should definitely steer clear.  I've owned an iPhone 6 for the past two years. When Apple released iOS 11 in early September, I was still using iOS 9, and my phone was still running like new.  I held off on upgrading to iOS 11 for as long as I could, worried about rumors I'd heard that it would decrease my outdated phone's perform...