Problem is most people are clueless. Probably some think that if you backup your iPad you can restore an older version of the OS... they are going to be very disappointed...
Fortunately the performance impact of updates is going to be less and less of an issue, especially for the pro and the air, as hardware gets more and more powerful creating more and more headroom for the software.
In terms of battery life, stand-by time is already pretty bad on iPad, so there is not much to lose in my opinion. Especially now that you can set a charge limit and leave your iPad plugged in when you don't use it.
I agree completely when it comes to the point that many still think you can downgrade. Well, I assume they will learn by fire, like some who update the oldest supported devices.
While I completely agree that performance is nowadays less of an issue (although it isn’t mitigated yet, I tried an iPhone 11 straight from iOS 14 to iOS 18 and even though I would describe performance as very decent, there was nevertheless a significant difference with iOS 14. Lag was present on some apps that did not lag on iOS 14), I completely disagree on battery life, with a caveat.
I understand your perspective. You are somebody who has a LOT of iPads, so you need the best possible standby battery life, and you don’t care much (if at all) about screen-on time. But now, put yourself in the shoes of somebody who has a single iPad. If screen-on time is destroyed by updates, the iPad loses a lot of usefulness. Grab a 10.5-inch iPad Pro on the latest supported version, and screen-on time is utter garbage. It has been destroyed.
After some years of inactivity (I had the Pencil but I didn’t use it), I have been using the Apple Pencil 2 with my iPad Air 5. If I may describe it like this myself, even with my honestly ridiculous efficiency (north of 20 hours of SOT on my Air 5 with light use, when many people have complained about battery life with M-series iPads with heavier use), the Pencil affects battery life significantly. In my experience, with similar use, the Pencil usage pattern with low brightness incurs a penalty of about 20%.
Imagine somebody who does not have easy access to a charger or likes to use their only iPad to draw somewhere or take notes with no power outlet nearby. If updates kill screen-on time, sometimes incurring reductions of as much as 60% vs the original iOS version, using the iPad becomes a power-outlet-hunting chore.
Battery life and actual SOT is something users don’t care much about when it is enough… but when it isn’t, it becomes the #1 issue for them.
On standby: M-series iPad Airs and Pros, as we have discussed, are notoriously bad on standby. But I also have the A16 iPad. By chance, I’ve been using the Air 5 100% of my iPad usage time for the past week. The 11th-gen iPad is at 91% after a full week, mirroring the results of the amazing early iPads, 2,3, and 4. So it seems that the M chipsets just cant idle well enoigh.
My Air 5 on it’s original iPadOS version (iPadOS 15) is ridiculously poor on standby. Overnight it dropped from 97% to 95%. The 11th-gen iPad didn’t move, of course.
This leads me to believe that the issue is solely the M chipsets. Trade-offs, obviously. You get the features and the performance but unless the usage pattern is super light, battery life suffers. People have been complaining about the difference in battery life of M-series iPads vs A-Series iPads since the M1 Pro.