What I’d like is app updates uncoupled from OS updates, so that older devices marooned on 16 can still get an updated Messages app without committing to a heavier-weight OS. How much overhead can come from emoji reactions to messages, for example?
I think that the main issue with that is Safari. An outdated browser just increasingly stops working. Mine is on iOS 12 and I have some issues with that. I don’t know how possible that is, they probably use frameworks unavailable on older iOS versions. At this point you start needing the whole update. Also, how far back do you go? Take Safari, which is the main app that stops working. Do you support... what? iPadOS 16 only? How about me on iOS 12? Because if it is just one major version it won’t make too much of a difference. I have an iPhone 6s on iOS 10. Do you allow me to update Safari too? I mean, it’s either very long-term support with the ability to downgrade to any version (including the original!) in case of issues, or it doesn’t make too much of a difference. Even then I think it’s not too possible for other apps, like I said, they probably use unavailable frameworks.
But yeah, I think people can tolerate apps not having features, but not apps failing. Safari fails, it stops loading websites, some even say “unsupported browser”, it starts being more and more useless.
General app compatibility (third-party) is an issue though, because that is the developer’s choice. On iOS 12, you pretty much need to download older versions. So, forget about downloading new apps, and you need to have the app on your purchased tab first, so unless you have downloaded it earlier, you need to have access to a supported device.
As far as I’ve seen, people who deliberately stay behind often give up due to this. You can adapt to almost any lack of features, but you can’t adapt to being unable to use what you need.
iOS 12 on my 9.7-inch iPad Pro is fine for me, but I know that it isn’t fine for everyone. These drawbacks often prevent people from staying behind, or in this case, eventually, sway them to upgrade once the device is unsupported.
For an iPad which is only used for content consumption? You’ll only struggle with Safari. The rest will work for a very long time on any version of iOS.