It's all relative and a matter of perceived speed. You can see it here with people calling the M1 slow...I agree that newer devices are better in terms of performance (not battery life), but still. Little things. Apple seemingly can’t - or won’t - solve keyboard lag. People report keyboard lag in the iPhone 11 on iOS 17.
I have two iPhone 6s. One on iOS 13, one on iOS 10. When I type quickly, the one on iOS 13 lags behind my finger. The one on iOS 10 does not. Occasionally dropped frames on the 11, too. Is it all the time? No. Is it device-breaking? No. But it bothers. It’s not that you can’t edit a video because it’s a slugfest. You can do it just fine, but little things start breaking that degrade the quality that you and I know Apple is capable of.
Let alone the complete obliteration of battery life and screen-on time that renders once full-two-day iPhones into unusable, stereotype-confirming “iPhones chase chargers all day” devices.
I think it’s more the quality of the experience dropping than “newer devices with the A12X being unusable”. Like you said, they aren’t, but if those little things start to break, coupled with a significant battery life loss, then people will - going to what the thread said - call older devices “glitchy and with awful battery life”. And the fault falls entirely on one aspect: Garbage updates, not processor speed.
That said: you are completely right on that aspect: RAM improvements help offset part of this. It is utterly undeniable that the experience on a fully updated 3rd-gen iPad Pro with the A12X will be infinitely better than a fully updated 9.7-inch iPad Pro, due to, like you said, increased RAM requirements. Flawless? No. Battery life? Still a lot worse. But the experience itself will be a complete 180 from a 2GB (or 1GB) device.
So much so, that five! major updates in, and people still call the 3rd-gen iPad Pro’s performance great! The typical, seemingly unsolvable battery life destruction, but at least the experience is great. Progress. Interestingly, there have been relatively frequent complaints about the iPhone Xʀ, with 3GB of RAM. Perhaps 4GB is the sweet spot for now. I wouldn’t know whether the Xʀ is as good, because even though I have one, mine is on iOS 12.
That said... are you sure that the cut-off isn’t the A12 instead of the A10X? I’ve read a lot of criticism in terms of performance when it comes to 2nd-gen iPad Pros, unlike 3rd-gen, and that’s ignoring the fact that battery life is complete garbage.
You have a M1 iPad so you know that speed it has and you can see the difference with the A9X.
A9X with 4GB handled things better but was still glitchy at times, A10X much less and it has not changed at all despite what people used to the faster pros that came later say.
A12X is clearly smoother and M1 is even smoother, but it's diminishing returns if we don't consider RAM.
The cutoff is where anyone puts it. I used to put it at A9X. iPadOS13 didn't change much from 12, if anything. I now put it at A10X. Maybe one day I'll put it higher. It's subjective.