Background downloads will change apps, sure. But does that really constitute a "complete re-think of how applications work"? It's the same feature, just implementable in a better way.
And people are in shock after years of iOS looking pretty much the same. When everyone actually looks at things objectively, they'll realise that their apps need to look a little different, behave a little different, but overall be pretty much the same apps they were before.
(and as an extra thought) you do not need to go far to imagine what apps with these new functionalities will be like - Android had them for a long time and Android apps are basically very similar to iOS ones.
There is a lot more new than just background downloads.
And it will change how developers think about their applications. Dynamic text, UIKit Dynamics, UIMotionEffect will change a lot as will new navigation/transition schemes.
I'm not up in the city right now, but from the few podcasts I've listened to - people are already commenting on how "dated" their app feels.
And - with the exception of the Mail/Calendar (Apple Favoritism, at its worst) and wacky "geofencing wakeup in the background techniques", all of our backgrounded Apps on iOS have basically gone to sleep permanently (until user pulled them back into the foreground) unless they were a GPS/VOIP/Music/Newstand app (and even some newsstand apps performed poorly with background downloads - I'm looking at you NYT).
In addition to the 3-D geometry (where I'm expecting lots of slide over sheets as a new metaphor) - I think all developers are right now considering how they can take advantage of an App that they can "wake up" remotely from a notification and perform activity with.