I'd seen some posts about iOS 7 and updating apps. Insta-delete via daringfireball. I think updating for the new iOS 7 look is a bit different than updating for the iPhone 5's screen.
We're of course updating ours. While the designers and people who notice things like which variation and weight of Helvetica is used in the text may delete apps, I really doubt that applies to the other 99% of the iOS users.
Every time Facebook updates something, there's a petition that goes around to change it back. Microsoft hasn't ever updated Windows without some level of backlash (especially Windows 8, but that's another story.) Android gets around it by not having any one upgrade, people eventually just get a new phone--and nobody is surprised when their new phone looks a bit different than their old phone.
We've had years of learning about what is a tab in iOS, what's a button, what's a toolbar. Now they look different. For a decent percent of people, those un-updated apps, that still look like iOS 6, will be familiar. There won't be that uncertainty about how the app will work. That chunk of people will be sad and upset when those apps later get updated for iOS 7.