You make some good points. My metric consists of my first smartphone, a 2016 SE, and the current 13 Mini bought a year ago. I guess that does qualify me to say a thing or two about the performance and battery regression, not replacing a phone every 1-2 years.
OG SE - Interesting that my SE didn't take a dive in performance immediately when upgrading to iOS 15.0, it (if recalling somewhat) around the .3 release. That when I noticed the KB lagging and only went downhill from there. After about 5 years I did replace the battery at Apple, as it really needed charging overnight, on the way to work, and work..permanently tethered and the European winter didn't help one bit. But there's the thing with this new battery and on the last iOS release. It just doesn't last! I keep it around as a spare, emergency phone on WiFi only and even after 1 year of a brand new battery, it dwindles to losing all charge on idle after 2 days or so.
What they did to A9 devices is horrible.
Firstly, they precluded people from keeping iOS 9. There is an activation bug on A9 devices on iOS 9, in which devices running this combo are deactivated, rebooted and forced to update. I lost iOS 9 on my 9.7-inch iPad Pro because of this.
Secondly, they pushed too far. Way too far. Whilst performance remains usable, battery life was obliterated. By iOS 13 A9 devices were gone and iOS 15 only added insult to injury. It’s my favourite device roster ever (if I could, I would still be using an iPhone 6s on iOS 9 and my 9.7-inch iPad Pro on iOS 9, too, but thanks to Apple pathetically forcing both out, I don’t), and it’s been obliterated.
On the other hand, I can speak from the other side: I have an iPhone 6s, which is my favourite iPhone ever... on iOS 10. Absolutely flawless. Original battery, battery life is like-new, perfect 3D Touch, perfect performance, it’s great.
But the current status is very sad, with A9 devices forced into constant battery replacements to get 5 hours of SOT because they otherwise can’t cope with the garbage that is iOS 15, whereas my 6s on iOS 10 happily sits there with flawless battery life and no battery replacements.
13 Mini - Bought a year ago with the last version of 15.x.x installed but was forced to load iOS16 when transferring data across from the SE. All was going well and for 9+ months it was still showing 100% battery health. I do recall it lasting 1 1/2 days per charge with no issues through all iterations of iOS16. Well, toward the end it was reducing itself to 1 day only but I was ok with that, also knowing it's a smaller battery. I even started a thread to gauge Mini user's experiences of slow downs , of those early adapters to iOS 17. There appeared to be no adverse effects and once my health went under 100% I was somewhat more carefree and upgraded also, BUT shouldn't have. 17.0 again, seemed fine, but after some further 17.x.x updates the KB lag became more obvious.
Actually one of the things that annoys me equally is that battery optimization only seemed to function properly in the first 4 months or so. I think I started a thread about this too but just cannot seem to have it work again since back then.
So yes, I'm annoyed about those slow downs, a new battery not lasting much past 1 year and a pattern (in my case and with a metric totaling two phones), that updating to the first xx.0 iOS version seemed ok but further updates crippling things more so.
Keyboard lag on a 13-series iPhone on iOS 17 is beyond absurd. You experience matches long-term patterns that users have reported: the first major version is fine, it starts worsening with the second major update. The 6s on iOS 10 is flawless, it isn’t on iOS 11. Battery life issues are absurd too, it’s too new.
Impact obviously varies, the iPhone 11 on iOS 15 is great, it isn’t on iOS 16, but if it isn’t the second one it’s the third. What I know, is that perhaps you can be confident to update to the first major version at best, but I’d definitely stop there. I wouldn’t update in the first place, but the first major version seems the safest of all.
Out of my four main devices, three are flawless: iPad Air 5 on iPadOS 15, iPhone Xʀ on iOS 12, iPhone 6s on iOS 10; and one is not: 9.7-inch iPad Pro, forced into iOS 12.
In my experience, and I don’t want to undermine anyone, but this is true, people on forums aren’t the best to ask about this, because everyone recommends updating anything.
If I were to ask “I have my iPhone Xʀ on iOS 12, should I update?”, the answer will inevitably be yes. iOS 17 would obliterate my iPhone Xʀ when compared to iOS 12. I’m used to perfection. I’ve used this phone as my main for over four years, I will notice everything and my opinion won’t be good. But people will say it’s fine if I ask.
What I want to say is, update expecting the device to worsen. If it doesn’t, you were lucky, but always update expecting keyboard lag to appear, dropped frames, and significantly worse battery life. If it doesn’t, then you were lucky and you can be happy, but if it does, at least you aren’t surprised and disappointed because you expected an update to be perfect when that is rarely the case.