Apple seems to have a knack for playing the long game, and I don't think they're stupid. I wouldn't be surprised if they began leaning on developers to create iOS apps that could legitimately replace their OS X counterparts. Adobe, whether you love them or hate them, are making inroads with apps like Sketch, Draw and Premiere Clip (etc). Granted, they're a weak substitute for the real thing today, but they have to develop around weaker processors, tight memory constraints, smaller displays and limited input options.
The iPad Pro, meanwhile, is getting a bit closer to notebook-class performance, it has a larger display and more robust input tools (i.e. Pencil and Smart Keyboard) than any iOS device previously. It's the closest an iOS device has been to being powerful enough that it could theoretically run desktop applications.
So, if we had developers creating more desktop-caliber iOS apps on one hand... and increasingly powerful devices on the other, then I could see us getting to a point of convergence in the future where iOS could legitimately replace OS X. A unified operating system that runs across an entire product lineup. In a sense, I'm describing what Microsoft is trying to do with Continuum... but having recently purchased (and subsequently returned) a Surface Book, I have less confidence in their ability to pull it off. I applaud them for trying, but I think they've essentially made their laboratory public. It wouldn't be surprising to learn that Apple is planning something similar (i.e. a unified operating system with responsive apps that adapt to the device they're being used on), but unlike Microsoft, would play their cards close to the vest until they'd perfected it.
I guess that's an extremely long-winded way of saying that I don't think the iPad Pro will ever run OS X. I actually think it more likely that OS X is retired.