I think iPadOS is a true OS. I am decidedly not in favor of putting macOS on the iPad (maybe as an app, but not a dual-boot thing). I love iPadOS, and use my iPad as my primary computer. But I do think iPadOS should continue to get more macOS features. I think a big thing that would go a long way for some would be a Catalyst type system that would make all apps on the Mac App Store available on the iPad App Store. Personally I’m happy with iPadOS in it’s current state, though I’d like to see a few changes like a more functional Status Bar, manual organization options for the App Library, and a few other minor things. And I do think it would be nice for Mac App Store apps to be added.
If putting macOS on the iPad, dual boot is the better option as long as it’s optional. Ship the iPad with iPadOS only and if the end user wants it, they can partition the storage and install macOS themselves.
Memory management on iPadOS seems like it will be problematic for “macOS as an app”.
Not that either scenario is likely to happen anyway.
As for the Mac App Store being available for iPad, before FCP for iPad, I thought there was a chance of it. Now though, the probability seems very low.
In any case, I’m fairly content with iPadOS so whatever Apple implements or not as far as MacOS/App Store support wouldn’t affect me much (unless it eats up a huge chunk of precious internal storage).