I have had the rather unpleasant experience of updates failing on vanilla iOS, so now you cant say you have never heard of this happening.
It does happen and it can put the fear of God (other supranatural entaties are available) in you when it does. There's nothing like having an expensive paperweight and an overwhelming urge to find a computer post haste to get it fixed!
The iOS 9.0 update failed for a lot of people, myself included, due to an issue where the code at the "Slide to Upgrade" prompt was unresponsive.
In fact, the fix for that update bug was the number one item in the list of 9.0.1 bug fixes:
iOS 9.0 also failed for me, and it was not the first upgrade to fail. Ever since 7.1 I have had problems with my iPhones or iPads that usually require a full restoration or even a device replacement. Before that, my iPhones and iPads were rock-solid. The problems have happened both with OTA updates or with iTunes-connected updates. The Apple Store Geniuses said there is something corrupted on my devices that gets carried over each update (and is not present on my wife's iPhone). What it is, nobody knows, but it does respond to device replacement at least for the first restoration/update. I've never jailbroken my phone. My only vice is that I have many apps on my devices (over 1300 apps organized in folders).
After failing to restore multiple times after a failed 9.0 update, I finally broke down and completely set up my iPhone 6 as new and downloaded only 150 apps that I consider the most essential for now. I am currently restoring my iPad Air (again) after updating to 9.1. The iTunes restoration failed (would not download apps no matter how much prompting I did), and I am doing an iCloud restoration that is taking many hours. I fear that I may have to set up the iPad as new also and lose years of organizing apps on the iPad.
I use a 16 GB phone. I actually went from a 32 GB 5S to a 16 GB 6 to a 16 GB 6S+. The availability of iCloud Photo Library allowed me to get back into a 16 GB phone quite comfortably with a few GB free. I'm pretty fastidious about deleting apps and games I no longer use and I use the cloud for photos, videos, and music. Even on a limited data plan (I dropped unlimited last year to gain tethering since I was only using 2-3 GB most months) I find this works well for me.
Also, there are loads of people who don't use their smartphone for 50% of their record collection, just a bunch of podcasts that get deleted, have no videos on it and don't download all the new free apps as long as they have some space left. Those who say you need 32 would be surprised at how many truly don't hit 13. There's a big world of people out there who like using the technology but don't get a thrill from morefasterbetter.