It is a very good move for Apple in all sense because:
- Their revamped UI design after the iOS 7 update has put them on the same level as Android. It has garnered great reviews for it's usability and responsiveness. So for quite a while, Apple can work on other issues without worrying about the design. Same goes for the various new features it has introduced, such as Apple Pay, Health etc.
- Similarly, in terms of timely updates, Apple is pretty much up to the mark. So they can divert their energy into this crucial task.
- It will prove to be a strong ground for developers who will be assured that their applications will be running trouble-free on a more stabilized platform. Happy customers=More Sales.
- There is nothing more important than customer satisfaction. An overall, better user experience is more important than a bunch of features prone to bugs and problems.
- In 2009, Mac's Snow Leopard OS has been praised due to it's stability and overall performance improvements. Apple would definitely want to work on those lines again.
The timeline set aside by Apple for this is a year. Do not expect big features from the iOS 9 but a problem-free, strongly worked-upon user interface is guaranteed.