But why does iOS even need to exist at all..... (in the future)?
OSX can be make to do everything iOS can do, it can operate in a full mode with a full UI for when it's running in, let's call it. computer mode.
The same OSX could easily put up a touch interface and close/hide aspects of itself and display a simple touch UI at any time.
The only reason for iOS to ever exist was due to the power needed to run it. If that power demand was never an issue there would never have been a need to create a simpler, stripped down OS in the 1st place.
I'm no iOS programmer, so perhaps we can ask those you are:
Is there anything iOS cannot do?
Could, people, if they wanted create full versions of the following apps?
Photoshop
AutoCAD
Maya
3DSMax
etc etc. And let's say you used the new stylus as the same as a mouse pointer.
Could iOS do it?
I get you. That's not really my point though. Yes, I do believe iOS has the potential to be as capable as OS X is. But that won't stop it from being iOS. The original idea behind iOS was that it was supposed to be a version of OS X built for the iPhone. And that's what I think it will end up being.
What I think will happen is that Apple will make one OS, but make a different version of it for each individual device. So the same functionality and capability on each device, but a different UI on each one.