I appreciate your position, and I'm not advising you to not update. I'm speaking for myself. I just shelled out a huge amount of money for an iPhone 7 Plus with128 GB storage. I plan on keeping it longer than three years. If someone has an iPhone 5 and the update has improved their phone, that's great, but I wouldn't keep updating it. They aren't meant to last forever, and as the subreddit I've posted shows, they seem to be meant to fail. If your phone is working great now, it should work that way for a long time. I wish I'd known that when I first got my 4s and updated it to iOS 7. It never worked the same afterward. For a while iOS 9 had made some improvement, especially with battery usage, but it never regained that new phone feel it had before the 7. It's now become just a beautiful but useless piece of hardware.
Also, a point that sticks with me is that Apple won't let you back out of an update if it affects your phone's performance. Not being able to opt out, other than ignoring the distracting red notification icon, also makes it difficult to think it's a neutral process.
Again, it's just my opinion, but it's one that's held by a lot of iPhone users, as the Reddit link shows.