It's not yet possible to jailbreak a device running i OS 11, though that's in the works.
The second and much larger group is people like me who have an older device (two more more models behind the current flagship) who are happy with its current performance.
I now need to charge my phone about three times a day, it shuts down without warning, and it crashes when I have too many apps running.
So it goes without saying that it might not be the best decision to upgrade to the newest version of i OS for everyone, security risks be damned.
Eventually, however, I gave in to pressure from friends and colleagues, plugged my phone in, and upgraded.