I would pay good money for an iPad Pro that could be connected to a hub with mouse, monitor, and keyboard and run Mac OS while 'docked', with the ability to unplug and use it as a tablet with the normal iOS interface. It seems like a good idea to me, which is exactly why it won't happen. Why sell one device that can do almost everything you need when you can artificially limit them for the sake of selling multiple products?
It might happen one day, but not anytime soon. Five(unlikely) to ten(more likely) years from now is probably my guess. It's going to take a windows-based competitor successfully doing it before Apple will feel much heat to do it.
I like the *concept* of the Microsoft Surface Pro, but in practice its execution is extremely lacking. The interface in 'tablet mode' is very clunky and cumbersome, and battery life was extremely poor. I owned a Surface Pro 4. Reviewers crowed about it getting 7-10 hours of battery life. I was getting about 3-3.5 with power saver on, brightness at minimum, bluetooth off, and running Explorer with one single tab open open. Contacted Microsoft tech support and they took a look at the power consumption and said it was normal. I complained some more and got a replacement unit. It was exactly the same. Everyone else I knew at the time who owned a Surface Pro 4 had a similarly abysmal battery life as well. I felt burned to the point I will never give Microsoft another penny for their tablets
Apple probably has what it takes the execute the concept well. There's technically speaking zero reason they couldn't achieve it by the end of this year with arrival of the ARM Macs and MacOS running on ARM, but until the competition catches up we'll probably have a long wait ahead.
As it stands, I've ordered an iPad and will order one of the new AS Macs when they become available. Personally, I don't see the case for thinking that will be much, if any, cannibalization of sales.