The thing is, barely no one buys an iPad AND a Macbook anyway. Meaning the cannibalization is small.
Everybody I know with an iPad also has a laptop or desktop computer and a smartphone. But the important point there is
iPad not
New $1300+ iPad Pro with Magic Keyboard. The "traditional" iPad had a clear niche between phone and laptop -
that's why cannibalisation is small.
However, in the last couple of years, thanks to the M-series chips, Apple have pushed the iPad Pro to
MacBook Pro levels of power, larger screens - and price - which makes it harder to justify an iPad Pro
and a low/mid-range MacBook, and makes it more desirable to run full MacOS and/or have a half decent keyboard. There are a handful of applications where touch or Pencil input really offers something different, but many things just work best with a keyboard and pointer. One problem with the iPad is that full MacOS Apps aren't going to be much use without a keyboard and pointer, while attaching a keyboard kinda ruins it as a tablet for handheld work or using the Pencil.
I wonder if Apple are going down the wrong road by making the "iPad Pro" effectively still a tablet with an optional (and pricey) "keyboard case" - most other manufacturers offer a "2 in 1" convertible touchscreen laptop for that market, with some neat mechanism for flipping the screen into laptop, tablet or "easel" mode. Apple could make a really powerful convertible using M-series chips and a bit of ingenuity in how the screen was attached.
I dont want two Lightroom libraries for example. So no, it’s not a simple task to to make sure every app uses the same file location which isnt currently shared by the sandboxed OSes.
I don't think the solution would be to have some sort of dual-boot or sandboxed
operating systems - MacOS can already run iPadOS and iOS Apps, the main restriction being that some mobile Apps just don't make sense without a touch screen or mobile device features like accelerometers or GPS - all of which, presumably, could be added to MacOS-on-iPad. Once you have a version of MacOS with proper iPad support, then iPadOS itself can go away on what would be turning in to the "Mac Convertible".
Of course, none of these things are a 5 minute hack, especially where they require action from 3rd party developers.
Currently, many don't allow their iPad apps to run on MacOS - whether that's to avoid cannibalising their native MacOS versions, because the UIs don't work well with keyboard-and-pointer, because they use mobile hardware features or - quite reasonably - because they don't want the extra chore of
testing and supporting everything on a range of Macs on top of the menagerie of mobile devices they have to support.