How exactly are you affected by an optional thing that is off by default and hidden away. Apple could force you to watch some videos that explain why this is dangerous and make you read all the warnings.
Some smart developers and designers could design this so grandma is not tricked to install malware, you could even require you take a quiz online.
You also have the Mac OS example, it is not locked down and the viruses and malware are not rampant.
Honest question , can you take 1 minute and consider thins, is there a large monetary incentive for Apple to lock down iOS?