I was hoping to see OS X on it as well, but at that point it's a Macbook Air without a touchscreen. So, since the iPad Pro wasn't launched with OS X, I think they're keeping the UX separate so it doesn't cut into sales of the other. That says to me that they're seeing 'Pro users' using both which equals more $$$$. So with that, I don't think they'll phase out OS X into something iOS-like.
Microsoft has also been a good study in pushing UI toward mobile, then bringing it back to a combo that's more practical for desktop users. I don't think they follow in their footsteps, unless to one-up them and show them how it's really done.
Eventually, it won't make sense to have the same size iPad with OS X and OS X lappy with touch, if iOS/browser apps are capable of the same thing, so it'll come down to UX, keeping that separate, plus how to get people to buy more of your devices.