I think you're looking for there to be a hard cut line in between these different bikes and different disciplines.
If the Roubaix is the a bike which you could put a slightly fatter tire on and go on some bumpy dirt roads/packed bike paths, the Diverge is already ready to do that, and can handle a little more rough terrain.
As a rider, if you're the type who almost never ventures off road, except for that one rare occasion you take a short cut through the path in the woods, Roubaix. If your the type who wants to ride road, but wishes you could use those off road shortcuts and hard packed dirt paths daily, Diverge. Although only because the Diverge exists. You really could ride the Roubaix on those other terrains if you wanted too.
A very good way to compare the Roubaix and the Diverge on paper, would be to use a Venn Diagram that has a huge center portion, and small outer portions.