I always find it amusing how fans of customizability seem to over-estimate the importance of dicking around with something the vast majority of people primarily use as a tool, a useful utility. If the defaults work well (which seems to be the case for iOS, even toddlers appear to be able to use it), that's already much, much better than something that sucks by default but can be customized to suck less.
That said, if you really care about customizability, you can always jailbreak your iPhone and do whatever you like, lots of customizations possible on jailbroken iPhones. Or just buy an Android phone if customizability is high on your list of priority features. Again, this is more about personal preference than anything else.
On a side note: Years ago when I was just starting to use Linux, I spent weeks customizing every aspect of the look & feel of the user interface. After a while I always got bored with what I had and started to get irritated by the various usability issues my customizations had introduced, so I started over. I went from FVWM to fluxbox, to Gnome, to KDE, back to Gnome, to XFCE and then back to Gnome again. The last time I switched to Gnome I stopped caring about customizations and simply stuck with the defaults, I had more interesting and important things going on in my life to spend time on, instead of wasting my time trying to be smarter than the people who designed the user interfaces I was using. I bought my first Mac running OS X and just used it the way Apple designed it, and never looked back. Since then I lost intereset in customizing my computers and phones altogether, realizing it's more like a hobby than actually making anything 'better', because 9 out of 10 times, you're only making things worse.