Yeah, it was a great run! It was longer than any of mine! I used a 6s on iOS 9 as my main phone 2016-2019. Upgraded to my current XŹ then, so 2019-present, and started using this 6s on iOS 10 in 2021. My XŹ is on iOS 12 and has been since I got it. Funny that the 6s on iOS 9 got forced into iOS 13 by Appleās activation bug. Both 6s were purchased the same day, one came on iOS 9 and this one on iOS 10. My 9.7-inch iPad Pro was also forced out of iOS 9. Had I known, I wouldāve updated everything to iOS 10, theyād still be there... unfortunately I donāt have a crystal ball! Well, at least I can use this 6s on iOS 10 and at least the iPad is on iOS 12, which as far as reports go, itās a lot better than iPadOS 13 onwards, even if it is worse in terms of battery life when compared to iOS 9. Glass half-full... like I said, thereās no way I couldāve known that that was going to happen.
Ultimately, and in spite of my persistent recommendations not to update, the whole point is for the device to be useful to the person who uses it, and another important factor is for the person to like the device. Your experience on iOS 9 started to break and you updated it, and thatās okay. Youāve stated that the device is more useful to you on iOS 15 than it was on iOS 9, and as long as you like it, then Iād say that you made the right call! Unless you want to keep it as a relic (which depends on whether you even care about that), keeping a device on a useless iOS version (to you) is ultimately debatable: yes, the device probably performs better on iOS 9 and 10 in every aspect... but it is useless on that version, so it works better but it canāt do anything. Yes, it will eventually encounter the same fate on iOS 15 and with far worse performance and battery life, but at least it gets some more years of full functionality. As you can see, Iām not completely blindfolded as to the benefits of updating, even if Iāll never do so myself, unless forced. Extremely long term though, keeping a device on an early version is always better: an iPhone 5s on iOS 7 or 8 is a lot better than one on iOS 12, and a device on iOS 12 isnāt all too perfect as far as support goes. There comes a point in which even an updated device is rendered obsolete, in which case an early version would be preferable.
I personally prefer to use something else and keep the device working flawlessly forever, but I know that not everybody prefers that, and frankly, who am I to say that thatās wrong? Maybe they canāt use something else and they need the device to be able to download a certain app. I can warn about the sure-fire performance and battery life issues that will inevitably arise, but if the quandary is between: a cool phone on an original iOS version (or an early one) that canāt do anything I want it to or a device that doesnāt work so well but does everything I need, at least for a while longer... you either skirt around that need (by using something else, like I did), or ultimately updating is all you can do.