Care to provide some examples of iOS Viruses and exploits that install themselves and are un-removable due to restricted user experience? I have never seen any of those before.
If you got an older iPhone, it's best not to open random PDFs anymore.
I was kind of thinking more of examples of malware where people were actually using the lockdown of the OS to stop users removing it, but hey I said exploits and that's an exploit.
I think CyanogenMods solution, where root access has to be enabled in the Developer Settings, which have to be enabled by tapping the Android version number 6 times, is the perfect solution: My mum would stop immediately doing such a complicated thing if she wanted to install something.
That seems to me to illustrate the point of a locked down environment fairly well...