Will devs update their apps for iOS7? The answer is a distinct "no".
The prevalent mood floating around is that, come this fall, iOS 7 is going to reset the playing field, and any dev who doesn't update their app to the new iOS 7 design guidelines is going to be left behind.
This is wrong.
First of all, let's take a trip down memory lane and remember how long it took devs to add support for the iPhone 5's longer screen. They had no life-threatening reason to, because apps still worked fine, just with black bars.
Then realize that, iOS 7 didn't change design, it reduced it: the entire UI is extreme-white, and instantly-recognizable UI elements have been replaced by text everywhere.
This is a fantastic design language for one developer's apps, (like Google did when they pre-empted iOS 7 with their Gmail, Google Maps, YouTube, and YouTube Capture apps) but terrible for an entire platform's worth of hundreds of thousands of apps.
By definition, conforming to iOS 7 will mean losing the ability to differentiate. How will they be able to, when every single one of their competitors will be white and minimalist, just like theirs?
So, now we have the same situation as the iPhone 5 screen situation, an even more dire one, at that: because, unlike the iPhone 5 screen situation, customers will not be demanding the devs for updates. iOS users have been happy with how iOS apps have looked for the last six years, and their tastes are not going to suddenly change just because Apple told them to (that would raise disturbing questions as to their volatility).
So, developers won't do anything. Except for some fringe design obsessives, the majority of devs will not adopt the iOS 7 design guidelines: especially Facebook, Instagram, Tumblr, and all the other big-name developers who won't do anything, as they always do, at least until their bottom-line is threatened, or a year later when iOS 8 is released.