iOS is not as indestructible as you seem to think, and definitely it’s not if it’s not updated properly.
Victims are also not as stupid as you seem to think. Obviously the email from the Nigerian prince notwithstanding, there are very sophisticated attacks which can inject code via ways you seem to be unable to comprehend.
That’s why they fix zero days exploits.
That’s why critical security updates automatically installing is a thing. That’s why they have an actual lockdown mode to further secure iOS.
As I said, it tends to be targeted, but who’s to say who the target is, and who’s to say who has the exploit and why they want to use it and against whom.
The absolute fact of the matter is, if one uses an out of date operating system, one is at risk, even if one doesn’t think they are.
I don’t think anyone saying that it’s ok to run out of date operating systems should be listened to with any seriousness. Sure, it’s up to you or anyone how systems and networks are run and secured, but it’s without a doubt, bad advice to suggest that it’s ‘nothing’.