I don't really understand the insistence on if an iPad Pro grows an inch or two, that suddenly Mac OS has any relevance. It's a touch interface device, Mac OS is not, IOS is. Mac OS is for Macbooks, Pros, and whatever, but for Mac devices. IOS is for iPads with the touch/utensil interface, Mac OS is isn't necessary, they've been clearly delineated as two different formats with possible crossover. They aren't meant to be the same though.
If I want to draw on a Mac, I need a tablet. If I want to mix on a Mac, I need an external device. If I want to edit videos, I need the keyboard & trackpad/mouse. Those are an addition to an already powerful device. I don't need those external devices with an iPad, but that of course comes with limitations, that keep getting diminished as time goes on.
Just because the increase in size of an iPad means more weight & length, it doesn't suddenly make it a Mac. It does on the other hand mean we do get an even more powerful iPad with bigger internals. There are already users who don't take their iPads out everywhere, but HAVE the option to if they want to, same as Macbook users. An increase in size may not make it the most portable device, but it doesn't drastically change what the iPad is. For most it just changes what it's used for & how.
Who knows, maybe the architects, engineers, and surgeons who already use iPads for their specific uses will really appreciate the extra real estate. That professional market maybe worth Apple's time. Just because the iPad grows a few inches it doesn't mean they can't find a table to safely rest it on though, and it doesn't mean Mac OS suddenly becomes important to them.