You really didn't get one of the points I made and keep talking about iOS 9....
Old iOS versions had better standby time and somewhat better screen on time, but that was the PAST.
I’ll omit the “somewhat” because it isn’t. My 9.7-inch iPad Pro is already 25% worse in terms of battery life on iOS 12 when compared to iOS 9, after Apple forced it out. An iPadOS 16 model is abhorrent, but okay.
So your "theory" of the "original OS version is miles better than any subsequent OS version in terms of battery life" is totally groundless now.
You buy a device on iPadOS 14 for instance, whether you update it or not, standby time, and battery life in general, is hardly impacted, and battery health degradation has essentially the same impact on iPadOS 14, 15, 15, 17...
Those are probably too new. The Air 5 debuted on iPadOS 15, I have mine there, but iPadOS 17 isn’t enough obliteration yet. Time will tell whether M1 iPads are also destroyed, but there are two categories: iPads that have been destroyed by updates (I’d say every iPad released before 2018, because I haven’t seen any data about the 3rd-gen iPad Pro, but presumably that’s included too, since the iPhone Xʀ has been severely affected. OP also claims it’s a lot worse); and iPads that will be destroyed by updates (everything else, unless Apple changes something).
That's the reality, and you keep repeting your theory with no first hand proof other that your selective bias when you read comments and only keep track of what kind of suits your theory (but while you have no proof yourself you ask for scientif proof from others here to prove wrong a theory you have zero data about)
I ask for a screenshot. If battery life were so good on updated devices, users wouldn’t be so reluctant to share their battery life. I’ve shared mine, for every device. I’ve shared what I got on iOS 9 and what I get on iOS 12, I’ve shared iPhone Xʀ screenshots. I have no intention of either defending or criticizing Apple for nothing. If it’s good, I’ll say it. If it’s poor, I’ll say it too.
I’ve tested an iPhone 8 on iOS 14. It’s the only iOS device I’ve tested that after three iOS updates, battery life was like-new. I would recommend any iPhone 8 user that struggles with compatibility on iOS 11 to update to iOS 14. But not 16. 16 is poor. Like I said, they always push too far. But if it’s good I’ll say it, I have no problems with that.
Also, everyone keeps misunderstanding this. It’s not “every iOS version is worse than the one that preceded it”, it’s “Apple keeps pushing devices way more than they should, so that eventually, devices are significantly worse WHEN COMPARED TO THE ORIGINAL VERSION”. I don’t care if iOS 14 is better than 13 on the 6s, because it’s worse than iOS 9. I don’t care if iOS 14 is as good as iOS 11 on the iPhone 8, because iOS 16 is a lot worse.
iOS 13 on the Xʀ is good. iOS 14 is good. iOS 15 is good. iOS 16 isn’t, iOS 17 is significantly worse. They always push too far.
On iPads, since this thread is about iPad battery replacements, the case remains. Apple offers a battery replacement that can only happen once, then the device degrades again and battery life goes from “significantly worse when compared to the original iOS version, but somewhat usable” to “permanently unusable”, and henceforth isn’t a viable solution.