Another issue that is important in relation to this, which is partly what I’ve said on the other thread, is what the user can tolerate and what the user requires. Like people have said, you can download older versions of many, many apps, even after the device loses support, should you choose to update is as far as it will go. If you update it, longevity for web browsing and app support extends massively, but performance and - especially nowadays - battery life, suffer a lot. Are you willing to use it like that? Are you willing to tolerate keyboard lag, occasional slowness, and worse battery life in order to use those apps and to have a good web browsing experience? I doubt the performance is bad on that model, though, so I reckon it will be good until the end of its update lifetime, even if not perfect. Even so, after it loses support, web browsing is still good. Barring a handful of websites, I can access almost anything on my 9.7-inch iPad Pro on iOS 12. I’ve encountered issues, of course, but nothing too major. The issues are there and I see them. I grab my Air 5 on iPadOS 15 and all of those issues are solved, so it is definitely the iOS version.
Should you choose to stop updating it at some point, are you willing to tolerate waning app support, existing apps’ incompatibility should they require updates to function, even faster web browsing degradation as websites stop working, with top-notch performance and battery life? Are your requirements static or do they evolve? If the former, you’ll be fine; if the latter… not so much.
I noticed something like this when Apple forced my 9.7-inch iPad Pro from iOS 9 to iOS 12. The only thing I “gained” from it was a severely worse battery life. My requirements are static, just content consumption, so I gained nothing by updating. I used it for the exact same things I used to use it on iOS 9, just with worse performance (nothing too unusable) and bad battery life. That’s because my needs are static. Should I be a user that downloads new apps left and right, I’d probably be forced to update it as far as it goes, and that’s a sure-fire way of obliterating both performance and battery life. I’d be able to web browse and use all of the new apps, however. Yes, websites stop working, but four iOS versions behind and I am not encountering anything major, yet.
Device longevity is, in my opinion, completely subjective and entirely dependent on needs. iOS support does not, in any way, determine longevity, again, in my opinion. Your mileage may vary.