Imagine this: your OS is so ugly that thousands of users go to extreme measures to change it even at risk of rendering their computer a pile a molten goo. Same with Windows. One would imagine that not one of the people who worked on the XP gui (mainly execs) would have been jumping for joy when over 100,000 users downloaded hte Aqua skin for XP. It just states that the gui was poorly done and everyone wants to be rid of it. Obviously not an image Apple wants to have. Not to mention that when theming gets easier themes become common place for novice users. Everytime an ugly theme is used its spreading that image... not to mention reducing recognizability.

People love Aqua. So much that the most used themes simply change Aqua slightly. Aqua was not hatched by some designer using ThemePark, it was done by a team of pro's. Simply put, without teamwork it would be impossible to create an interface as widely acceptable as Aqua.

I don't think Aqua will remain the same forever. Its very likely that it'll eventually undergo major changes to its look... probably to match hardware.