I really hate that they demand Wifi - I never have Wifi, but I have an unlimited data plan that is well good for multiple terabytes.
The person who buys the phone is one customer. Another customer is the cell phone networks, who don't much want to use their airtime to distribute OS patches, which use a lot of bandwidth but are almost entirely invisible to users.
Rather than, you know, a vulnerable device behind on updates that isn't backupable to iCloud in any way.
It should probably have to be turned on explicitly, because a lot of people are still on spendy OTA plans, but it should be an option.
Battery crapping out is still a major taboo with iOS updates, I believe.