Finally Apple unveiled a whole new world on which it was working on since a long time to showcase on its WWDC 13. Therefore, today Apple finally showed off the products on which it was working to make them look different from their preceding versions. While showing off with new and innovative products, Apple demoed iOS 7 with a complete new design, UI, icons, and features that are exclusively available in iOS 7. No preceding version of iOS contains those features, UI, icons, and much more. Apple no doubt did a great makeover of iOS 7 for iPhone, iPod, and the iPad.
In order to enjoy lot of new features in iOS 7, Apple dropped the support of iPhone 3GS and iPod Touch 4 because they were too old to support iOS 7. The support of iOS 7 starts from iPhone 4 to iPhone 5, iPad 2 to iPad 4, and iPad Mini as well.
Dropping the support of iPhone 3GS and iPod Touch 4 for iOS 7 seems to be a legit decision because these devices lag on the latest iOS 6.1.3 version. If Apple had rolled out iOS 7 for them, these devices would have been dead for sure. Therefore, Apple dropped the support of iOS 7 from iPhone 3GS and iPod Touch 4. Where as, iPhone 4 and beyond will support the latest iOS 7 version.
People owning an iPod Touch4 or an iPhone 3GS would need to update their devices to the latest Apple gadget available for them on contract or off contract. However, it is not mandatory to update to iPhone 4 or beyond. Only update if you want to enjoy a newly designed iOS version.