Why Doesn't Apple Use the iPad as a “Touch Bar on Steroids” for the Mac?
The Touch Bar has some nice potential, but the drawbacks to such a thing are numerous and obvious -- what if you want to use an external keyboard? How will this work on iMacs -- will they sell a $200 standalone keyboard with a Touch Bar?
All of this is a bit maddening because many of us already own a fantastic Apple-branded touch device: an iPad.
For some tasks I'd love to have an iPad sitting next to my keyboard as an aux input device for macOS. Photoshop and its ilk come to mind immediately. Jog controls for video editing... music creation apps, etc.
There are existing iOS apps that allow your iOS device to function as a generic trackpad or external display for macOS, and things like Astropad that take it a step farther. But in order to really take off, in my opinion this is something that would need to be baked into macOS and XCode themselves.