The main issue you have is the OS has security holes, as do the core apps like Safari and Messages, and many of the third party ones will too. Apart from that, it’s all good.
The security argument falls apart immediately. Why? Because what happens when Apple stops supporting the device anyway? What if I don’t want to or can’t buy another, before you say “buy a new one”? Do I have to no longer use my iPad? No way. I’ll keep it in the best possible version Apple allows me (i.e., the earliest), and it will last a while like that.
In fact, I’d argue that the main issue is a different one (or a couple):
-App support: The #1 staying-on-earlier-versions killer. I’ve seen many, many, many people, countless people, stay on early versions of iOS for years until, like I said, they don’t want to or can’t buy a new one, and some apps stop working (because some require the latest versions which in turn require a newer version of iOS) or they cannot download new ones because the initial version requirement is outright higher and they don’t have a device to download it first and then download an earlier version. It is impossible to solve without updating and tough to put up with if enough versions of iOS have passed and the user is a frequent new app user.
-Features: This one is completely irrelevant for me because I frankly don’t care, but some people really like using the new features.
-Safari: While it can be mitigated through the use of a third-party browser, I like to use Safari and its usefulness diminishes a lot. Many, many websites don’t load if you stay behind enough. I’m on iOS 10 on my iPhone 6s and Safari works half the time, it’s not great. This is obviously because Safari updates are bundled with iOS.
Edit: both App Support and Safari have the same qualms even when updating: they eventually stop working perfectly regardless, and you have a far more crippled device that if you were to leave it on its original iOS version.
Even if usefulness is greatly reduced, what would you prefer, four years from now, or five? An iPad Air 2 on iOS 8 or an iPad Air 2 on iPadOS 15? The former will be perfect for basic apps, the latter won’t be, Safari will work poorly on both, and security will be poor on both. Give it enough time, and any argument in favour of updating is nullified.
Yeah, you can say, you gain a lot of life if you update, even at the expense of battery life and performance, and then you will need to upgrade anyway, either because the iOS version is too old (iOS 8), and can’t run anything other than the most basic tasks, or because the device works too poorly AND the iOS version Will also be too old (albeit a lot newer). So, you gain a little time at the massive expense of both performance and battery life; otherwise, you lose some functions quickly (or more quickly than a fully updated device), but the device remains flawless for its entire lifetime.
