My phone has virtually everything themed, and has dozens of tweaks, and you have to remember these are running on top of Apple's stock OS, and my phone works just as good as a stocker. So I don't see why or how themes, which are nothing more than the same UI elements but with different colors or patterns would cause any problems, especially if they were built into the OS by Apple. A theme store would rake in the dough as well.
Choice is good, After 4 years I'm bored of it, it looks exactly the same as it did on my old 3G! Time for a change, they don't have to completely re-do everything, just a re-fresh would do, some new colors (the dull faded grey and baby blues everywhere look dated and stale) a more modern look.
And of course there are still glaring holes in iOS, like the fact you still cannot reply to a text message without interrupting what you're doing and going into the SMS app. Widgets would be nice, and for all that is holy, that stupid weather icon that is stuck on 73 degrees and sunny is a joke.
So there is plenty they can do without hurting anything, it seems the people bored with iOS are those of us that have been using it for years now, Apple needs to do something to keep us around, there are only tens of millions of us!