I didn’t say theming is useless. I said theming is bad for people who don’t know what they’re doing, which is almost everybody.
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You can’t idiot proof a customization system because the phone can not determine what is and isn’t good design. Technical considerations can be made idiot proof because a program can determine what is and isn’t valid.
Customization would screw up the device, in that 99% of the people will use them to make their phones look horrible. It reduces brand prestige in a subconscious way. If most of the Apple phones you have seen looked decent, then you get a positive image of Apple. Whereas if most of the Apple phones you see have cheap lame themes on them, it makes the brand seem cheap and lame.
What in the world are you babbling about? We are simply talking about Apple providing options to change icon sizes by selecting one of a few preset options, changing the grid size with a few preset options, and possibly enabling a tick box to enable/disable icon labels. Apple absolutely can make those simple customization options idiot proof and they could probably do it all in a day and push it through with a minimal OS update. It's not nearly as complicated as you are making it out to be and your continued random comparison to cars is hilariously irrelevant.
In addition, your comment about "brand prestige" immediately goes out the window when we consider what the majority of iPhone home screens look like these days. Essentially... people using obnoxious pictures of themselves and their pets as their wallpapers while filling their home screens to the max with apps like Doodle Jump, Snapchat, Pintrest, facebook, Twitter etc.
Most of the iPhone home screens I've seen look absolutely terrible and cheapen the brand appearance way more than the simple customization features most of us are referring to.