iOS 1 was easiest OS ever and most stable too. It was very restricted and you couldn’t do some things with it. No background, no apps, no even copy and paste. A lot of people like it, it changed how smartphones work.
People wanted more, slowly iOS started gain PC and Android like capabilities. With each release it involved we saw more and more power features, bad performance on old hardware and some complications with UI. Still I think Apple manages this three aspects very well.
Things changed a lot with iOS 8 and Xcode 6. We have so many capabilities now. Imagine how would be easy develop apps for iPhone 3G S today. Developer tools involved a lot. iOS is much more capable, there is no more major restrictions and still relatively easy to use.
But nothing comes free. First people wanted more features and now they complaining that iPhone is slower, unstable and even crashing. But we forgot that everything can not be perfect. Progress takes some time and Apple has a lot of mistakes to make. To learn and evolve. All they can do is continue to balance while evolving every year.