That's because it's essentially the same as all the previous versions of iOS. It's going to take more than a horrible skin and a few bugs to get people to stop using the iPhone. It will however, affect people's perception of Apple. Instead of being known for making the best products, they will slowly degrade to just another company. There was a report that came out a few weeks ago showing that customer satisfaction for iPad and iPhone had dropped. Hmm.. I wonder what's causing that.
People are so quick to point out Apple's sales when trying to justify Tim Cook's reign at Apple. What they don't understand is that Apple have not created any new products since Steve Jobs died. All they have done is update existing products.. The current iPhones, iPads, MacBooks, iMacs etc.. All have the same form factor and essentially the same UI as the ones being sold when Steve was around. These are great products and it will take more than horrible UI changes like iOS 7 and Yosemite to stop people buying them.
The real test for Apple is when they create something entirely new; something that didn't exist in any form when Steve Jobs was around. The Apple Watch is that product. Let's see how well it does...