History has shown that eventually devices are obsolete. It is impossible, unless product and iOS releases just stop. The iPhone 5s was considered massively futureproof, and today, while usable, it isn’t perfect.
That said, many people use iPads for content consumption. For that, it will be fine for a very long time. The nearly 10-year-old iPad Air 1 is okay for content consumption. It can read books, it can play TV shows, it can access apps like YouTube, and a long list of etc. I don’t see that changing. Yes, Safari will increasingly struggle. That’s the only content consumption-related aspect that goes obsolete. Otherwise, it will be fine.
Battery life is a problem if extremely obliterated by iOS updates. Updated 32-bit iPads are fine, and while newer iPads do struggle, if they’re kept on an early enough version of iOS, they’ll be fine.
If usage is basic enough, it will be fine for many, many years. Forever is a little too much, but “usable for a very long time” is definitely not an exaggeration.
The issue today, like I hinted at before, even for basic content consumption, is battery life when updated. 1st-gen iPad Pros on iPadOS 16 have quite poor battery life, with many people with 7-8-year-old iPads reporting extremely poor battery life, in the realm of maybe 4-5 hours. For content consumption, if a device on iOS 12 is fine, I see no reason to think that first-gen iPad Pros will struggle at all, barring battery life.
Why would a person with a 1st-gen iPad Pro who wants it to draw and for content consumption need to upgrade at all? It will work for that. Newer iPads will do more, more layers for example, but if the user is fine with what their device can do, it will be fine for a very, very long time.
I guess there is a golden rule for this: static requirements. iPads will be good for a massively long time. Like I said, barring web browsing, a 1st-gen iPad Pro user only needs to worry about battery life if updated. Otherwise? It will always be able to draw, to read books, to watch TV shows (on apps that work on older versions like Netflix), it will always be useful for note-taking, and many, many other things. That will not change when the device is on an older version of iOS, and that will not change when new models are released. A 1st-gen iPad Pro user will always be able to do that. With limitations compared to new devices? Yes. With limitations compared to itself? No. That device will always be able to draw and write in Procreate and Notability with current capabilities, it will always be able to watch YouTube and TV shows, it will always be able to read books.
The only limitation is app support when apps require updates, and those updates are forced. Apps that won’t allow older versions to work is the only time in which an iPad on an older version of iOS will see a regression in terms of capabilities (apart from web browsing). It won’t be able to do more (like newer devices), but for tasks that respect static versions, the iPad will be useful until the user gets tired of it.
I do not know how newer iPads will evolve. History has only showed its unpredictability: updated 32-bit devices are useless because they’re pathetically slow, and newer devices fare a lot better in terms of performance. Current devices at their update limit are a lot better, but they suffer in terms of battery life. It is impossible to know what the future holds, but as of now, we can say that for fairly basic and static requirements, barring specific exceptions, iPads are extremely long-lasting.
As far as battery life deteriorating... if conditions support longevity, will it? How long would it take? I think we can all agree that current updated iPads suffer a lot in that regard. But for the sake of this argument, I will give two examples. While I have mentioned this before, I think it only adds to the point:
I have two devices which are my two favourite devices ever: an iPhone 6s and a 9.7-inch iPad Pro. The former runs iOS 10; the latter, iOS 12, after being forced out of iOS 9 due to Apple’s A9 activation bug. My iPhone 6s’ battery life, after nearly 7 years, 1400 cycles, and with 63% health, is like-new. 7 hours of screen-on time on its original battery running iOS 10. My 9.7-inch iPad Pro got 13-14 hours of screen-on time on iOS 9. It dropped to 10-11 hours immediately after it was forced into iOS 12 (I did not update it willingly). This happened 3.5 years ago. Battery life today remains exactly the same as it was immediately after it was forced, which means it maintains the exact same battery life it had when it was new, of course, iOS updates-induced drain notwithstanding. This is the important part, and it requires an early enough version of iOS so as not to be obliterated regardless of battery health, but I ask... will it ever see a degradation? If an iPhone 6s is flawless 7 years in with a small battery, will an iPad ever degrade? What would it take? It is certainly an interesting aspect, and my answer is that I don’t know. Would 3000 cycles be enough? Three times the spec sheet is not enough to affect an iPhone 6s on iOS 10, I’d reckon it isn’t enough to affect an iPad with a far larger battery either. So, arguably, battery life shouldn’t be a problem. How much time would the user need to really degrade an iPad on a half-decent version of iOS?