i too believe apple wants one single OS. the only difference between iOS and OSX is touch vs mouse. but just like a laptop, its just an option... touchpad or mouse.
so i believe one day we'll see one single OS. what's the point of having two?
The point of having two is that middle ground products never work well.
The trade off are too much for the product to bear and it just ends up unclear.
To me the question is why merge them?
OS X provides enough guidance to get out of your way and let you get on with stuff, plus has freedom to let you try almost anything you can think of.
iOS is tight and nibble so you can get in get stuff done then leave it to the device to deal with it. It shouldn't let anything get in your way.
Both get to take advantage of common ground which gives them all the advantage of a merged OS but they aren't tied to it if they need to push away and better serve the customer.