I think that would be technically and mechanically more complex, with possibly different operating systems for the different segments of the display to avoid unwanted interactions. It also means that the secondary display could not be sold separately as an upgrade. If you could buy the bike with just the current TFT instrument panel, then the only way to upgrade would be to remove that and replace with a combined unit rather than just add a secondary display.
Not making use of a phone and Apple Carplay or Google Auto to drive a secondary screen also has other drawbacks.
For a start it means BMW, who are not primarily a software house, if they are not going to piggyback off existing smartphone technology, are going to have to devise and maintain complex software to manage the required functionality for satnav, music playing, telephony, etc. Even if they do maintain the use of an app connected smartphone to drive these functions, we all know how poor they have been at making the current TFT work acceptably with many phones and headsets, so I wouldn't want to be an early adopter of such a system.
The other issue is that in this scenario the consumer only gets to use such functionality as BMW can either build into this mega display, or alternatively can get their in-house phone app to adequately support on that display - so don't expect to use your favourite satnav app on your phone, or your favourite music player app, etc, and have it passed through to the display by BMW's app.
In contrast, a separate secondary display dedicated to providing support for the automotive interfaces from Apple and Google would give consumers much more flexibility and choice in what they want to use, and these have massive resources behind their development and are likely to become the industry standards. I don't see BMW going it alone being able to compete with that, but I could be wrong - what do they use in their cars for this sort of thing?
Fred
Lots of good points here.
In their cars BMW use their own software, but Auto Car play or Apples whatever play may be used.
I don't know what they will choose to do, but what ever it will be, I feel fairly certain that they will not call me for advice, so I am looking forward to see their choice.