There's a great short post on the Tapadoo site called "iOS 7. Move now, or be an insta-delete." As a dev shop with outside clients, they see more clearly than most that iOS 7 is a seismic shift for Apple's mobile platform. This will be more emphatic than the rapid turnover we saw with apps being updated for the iPhone 5 screen's new aspect ratio; apps that don't get refreshed for iOS 7 will seem antiquated to a majority of Apple's mobile customers come this fall.
Those customers update quickly, even if the flashiest new features might not make it all the way down through the installed base. iOS 7 sets the stage for the next several years. Even the corner radius on app icons is slightly different in 7 than 6 -- so older apps will be slightly, but noticeably out-of-date just sitting there on the home screen.
As I said after WWDC, iOS 7 has a ton of great features. Developers, many wary at first (the rumor mill had fanned the flames of fear by calling the new OS "polarizing"), have come to embrace the changes. I have yet to see a developer give up on Apple because of the new aspects in iOS 7. In fact, I have seen fears allayed, and much excitement over the new OS.
As Tapadoo points out, "If you have [apps] in the store, we'd highly recommend looking at modernising them pretty quick." I concur.