Clearly, you are not a programmer. It?s doubtful that the folks at Apple rewrote every single line of the OS... but whenever you start rewriting and changing the internals, stuff breaks. The code that makes up iOS (any version) is huge, and complex. There are thousands of "moving parts", and whenever anything is changed, unintended consequences follow. What often times appears to be a simple "tweak" can have major (negative) impact on that or other areas of the code. When redesigning/rewriting an OS, very few of the changes are simple tweaks.
There are new features, a new SDK, and support for new, yet to be introduced hardware, etc. Stuff happens, and that?s why we test, retest, and test again...