Perhaps but the lines have a lot of overlap now—more next month. It would make sense to replace the iMac Pro with such a machine but I'm ok if that doesn't happen. I'll go out on a limb and predict that the Mini gets a decent GPU first (could happen, right?).
How old does one need to be to remember all the armchair experts and PC centric rags who screamed that no one would buy the first iMac because it wasn't expandable except for RAM? And then Apple sold a million.
I think the reality is that most folks prefer the AIO. Those who don't are a niche market. Ok, there are thousands who would argue against that here—out of the many millions who buy iMacs, do what they do and would never think of looking for MacRumors.
I do wonder if the iMac Pro would have existed had Apple not made the misstep we call the 6.1 Mac Pro.