No, I have always said that the key part of my theory is “if updated far enough”. Some major iOS versions (typically the first one) are fine. If there’s a difference, it’s negligible. Specific amounts (which is why I don’t give any) of major iOS updates required varies. For some devices, the first one is abhorrent. For others, you can update it up to three times and they’re fine. Once you pass that threshold, though, it’s all downhill from there.
Funnily enough, I’m unsure about extreme degradation on some halfway updated devices. Take my 9.7-inch iPad Pro. Assume that at some point battery life really decreases. Say, 40% health, 30%, whatever, take whatever number you like. The question will always be there: ”Wait. Would it be better on iOS 9? If so, how much?” So, perhaps they’re mostly fine on a version, but if degraded far enough, they suffer, and they wouldn’t if they weren’t updated at all.
Regardless, the chances of an iPad being degraded to such a degree are rather low. Like I said, the battery typically outlasts the version obsolescence itself. You can degrade, say, a 9.7 or a 10.5-inch iPad Pro if you’re a heavy enough user (partly thanks to Apple’s long support lifespan: if you require updates, the longer they’re supported the longer you will use them), but from the other side, I’ll never be able to tell you for sure that it wouldn’t be like-new if the device were seven iOS versions behind.