Eventually, yes, most users upgrade. But Apple's stats showed about 6% of users still on iOS 5 a full year after iOS 6 was released. I'd imagine we'll see similar numbers with users going from iOS 6 to 7 - lots of them won't upgrade for a full year, and some will never upgrade. Mixpanel is currently showing 36% of users on iOS 6 - that's not an insignificant number. I'll be supporting iOS 6 for probably another year.