I also agree with Apple's implicit claim that if iOS users could sideload apps, millions of idiot iOS users would get their devices owned after they followed some "follow these seven steps to get free $POPULAR_MOBILE_GAME tokens!" guide they found on the web, making the platform less trustworthy overall.
Apple makes a good argument that buying an iPhone is also buying, in a sense, a remote managed security service for the device at the same time. The net effect of this is that millions of people now have devices mostly free of the most egregious malware (and it's limited to just spyware, delivered via the App Store). For most users, this is a better state of affairs (at least in peacetime, or outside of China/Vietnam/Russia/etc).