I think the biggest problem comes from supporting external displays. For me to fully swap to iOS as a development platform (ignoring missing software currently) I would need to be able to plug into a larger display at some point, and have a way of controlling apps on that display without using touch at all.

Once you get to that point, though, it seems like what you're really looking for is a computer and not a mobile device.

Perhaps what's missing is not an iPad with mouse support, but a Mac with a more Surface-like form factor.

I'm posting using a wireless mouse and USB keyboard from my Android desktop, a 4K tv box.

Android does support mouse input; Apple can at any time release a typecover for iPad with a trackpad. They choose not to.

iOS already supports two displays with AirPlay. You can have a separate image on the device and the display.

I have no problem with a mouse solution that's only active in textareas. But, as others note, if the external display is a real option then a mouse becomes necessary. Personally, I still doubt the iPad will drive external displays.

The latter of which already exists: force touch anywhere on the keyboard to drag the text cursor around.