It certainly will _feel_ persistent if you're successfully attacked with this technique.
If your iOS software is swapped out for a version with a backdoor, then the attacker will have collected your passwords and authentication tokens to services you use. If you reboot to clear the backdoor (and let's be honest: no one reboots their phones), then you won't also "clear" your attacker's memory of all your passwords.
If your iOS software is swapped out for a version with a backdoor, then the attacker will have collected your passwords and authentication tokens to services you use. If you reboot to clear the backdoor (and let's be honest: no one reboots their phones), then you won't also "clear" your attacker's memory of all your passwords.