If anything, it makes the iPhone safer. If only one website renderer is used, you only need to find a zero-day in that one renderer to potentially infect all iPhones. Now that other web engines are going to be permitted, attackers will have to contend with multiple web engines. And you as user can choose to use a smaller web engine like Gecko in order to decrease the likelihood of being successfully attacked.
If anything, it makes the iPhone safer. If only one website renderer is used, you only need to find a zero-day in that one renderer to potentially infect all iPhones. Now that other web engines are going to be permitted, attackers will have to contend with multiple web engines. And you as user can choose to use a smaller web engine like Gecko in order to decrease the likelihood of being successfully attacked.