The Iconfactory's Gedeon Maheux, in a piece entitled "Watershed Moment":
[H]ow willing would you be to re-purchase your favorite apps if they are optimized for iOS 7? Look at your device’s home screen and go down the list of apps you use most and ask yourself if you could live without it once you upgrade. I think that most users (at least those that matter to developers) would answer that they would gladly pay again if it means having the latest and greatest version of their favorite apps, at least I would hope so.
As I wrote yesterday, I'm more than happy to pay again for great apps — you should, too.
iOS 7 represents such a huge shift in the operating system's design language (internally and externally) that I hope users realize sooner than later that holding on to their "legacy" apps because they don't want to pay more money for newer, better versions is a mistake. For the best in performance and experience, it's best to pay up for iOS 7-optimized software.