Right. I think the apps will probably get updates to take advantage of the new hardware. What I am saying is, I don't think the core OS will be that different on the Pro vs the iPad 8. We MIGHT see full screen support for monitor output, and some other minor features like that, but I don't think we'll see a bunch of major upgrades that will substantially differentiate the core OS from the other models. Hardware features are one thing, but to design an entire OS around one model having a more powerful chip or more RAM is very un-Apple like.Just because an app can run on both iPad 8 and iPad Pro doesn't mean the experience will be the same. For example, games allow higher quality settings on newer/higher end iPad versus older/lower end models.
Sure, the iPad 8 will run iPadOS 15 (I'm guessing at least the iPad 6 as well). However, it doesn't mean they'll have the same feature set as what's available on newer/higher end models. Apple has restricted features before based on hardware (Night Shift, content blockers, multitasking, etc).
Then again, they DID just release lossless audio for their music service despite their most expensive pair of headphones (even wired) and premium speaker not supporting it, so I guess anything is possible.