I’m not sure a one-year-old iPad can be degraded, but regardless, the iPad should be used in outstandingly poor conditions in order to suffer that quickly. I mean, I haven’t tried it obviously, but it would be interesting to see. Use an iPad on its original iOS version for 1 year in extremely hot environments with no care. I don’t know how that would turn out, but the key point is, you need extraordinarily poor conditions for any iPad on its original iOS version to suffer. Give it a little care, and it will be fine for a massively long time.
The original version does have the best battery life, always, and while updates can be harmless, they’re typically on the device’s first major version. The second is already pushing it. Sure, at first it can be minor (a two-year-old iPad won’t be obliterated by iOS updates, not that quickly).
Note that I am not talking about point updates, those can improve relative to older point versions of the same major versions.
Exclude extraordinarily poor treatment and conditions, and an iPad on its original iOS version with normal use should be fine... forever in iPad terms. And by this I mean, until the iOS version is too obsolete to be used. I’m sure they’ll still work for something though. An iPad Air 2 on iOS 8 would probably work for video streaming like YouTube and Netflix just fine, with very good battery life. Yes, practically nothing else would work, but the iOS version will be mostly obsolete before the battery suffers if kept on the original iOS version and treated “kindly”.
Still, I admit that I have one thing I need to know about this aspect: with very heavy usage throughout many years, how would an iPad fare, even if not updated? How would an iPad Air 2 on iOS 8, with proper care (avoiding heat and not doing something like keeping it plugged in 24/7), fare after 10 years and, say, 2500 cycles? I don’t know, because nobody does that. All I can say is, I wouldn’t be surprised if it were very decent. Nobody can prove that because older devices are all updated which excludes them from any serious consideration.
I’m not a heavy user and my oldest iPad has sadly been forced out of its original version (honestly, screw Apple for that abhorrent bug), but my 9.7-inch iPad Pro on iOS 12, 7 years after purchase and 4 years after being forced out... has the exact same battery life it had when it was originally forced out of iOS 9 back in September 2019, which means, it would probably have the same battery life if it were on iOS 9 (which again, it would be barring Apple’s pathetic bug).