Oh wait, my iPhone 35s doesn't support iOS 7 at all (for these reasons). But I know people with iPhone 4 who don't upgrade because iOS 7 lags too much even on iPhone 4.
Of course we gotta update our devices once in a while, but for those of us who don't live in the US and buy them unsubsidized, it costs some money. All I'm saying is, there are some reasons people won't upgrade right away even if they'd hell like to. Whether to support them for a while is up to you of course.
edit: and there's this iPod Touch thing of course.