The problem with an AIO is, it can only be as good as when it was released.
The maxed out iMac 5K 2015 when new was considered a beast at that time, and somewhat an adequate "Mac Pro" replacement if you don't mind using the internal 27" screen. The problem is it gives almost zero flexibility: CPU throttling due to lack of air room, soldered / hard to access drives, odd speed of SO-DIMM DDR3, TB2 being the best available I/O etc. It is now only 2 years and we already are at DDR4/TB3, and if either the screen or the computer itself breaks the whole unit it at a halt. A lab or studio with a room of 10 iMac 5K is much less likely to remain productive than 10 2012 Mac Pro's.
So if the projected "Mac Pro 2017" seriously does end up having a permanent screen attached, it will by default be an even less "Pro"machine than the 2013 nMP. If we are stuck with a choice between an MBP and an iMac then we may as well just use the MBP for portability factor. As ridiculous as we know, this is how Phil Shiller justified showing that photo of two 5K monitors attaching to the tbMBP 15" with a RED next to it, alluring it being a workstation replacement setup.