Note that Apple now keeps the previous version of the app around for users on older OSes. Of course they won't get any of the upgraded features, but at least there's still something there for them - and you won't have to worry about all that messy code for supporting multiple versions.
My next app updates will also be iOS7 only, since I know the iOS6 version will still be around. Makes it much easier to decide to use newer features.
One open issue is that from time to time a breaking change to our API is needed and a force-upgrade is forced on the app users. We try to avoid it, but it happens.
The next time this happens, it's really goodbye iOS 6 users.
Slightly off topic here. I'm a bit baffled by that app. It's a fugly - it seems like a pre ios 7 app that was resurrected in a few days and isn't very Appley at all. I wouldn't call is skeuomorphic, but it isn't the new(ish) flat look. The main screen's background just looks like someone forgot to include the artwork. Its a flat grey though, so must have been chosen. The clunky FAQ page isn't nice either, and a straight link to a web page would have been nicer IMHO.
Apple only offers older versions of your app to users that have previously purchased it. New users will still get the traditional "this app requires iOS 7" failure message.
I believe you can purchase apps you haven't purchased before. The writer of the article I read on this tried it. I've downloaded free apps whose latest versions weren't compatible on my device, and got the older version that works for my old (iOS 3.2) device a couple of months ago.
My next app updates will also be iOS7 only, since I know the iOS6 version will still be around. Makes it much easier to decide to use newer features.