I know the 6D series sometimes gets called the full frame Rebel series, but by any reasonable assessment the 6D series cameras are actually quite high end cameras, and a substantial step up from a Rebel class camera. For instance, 6D had a largely magnesium alloy body and 6D II still has a good quality body (aluminium alloy impregnated with glass fibire I believe I read), a body jsut a bit smaller and lgiher than the 5D series, pretty good ergonomics (yeah, the lack of joystick is not great!), a top plate for camera information and a level of weather sealing, etc.Why do you not choose to associate this with the 6D since full frame to full frame is apples to apples and Rebel to FF is a very far cry with the XXD line a much more tangible step closer if any crop is.
So, assuming the R6 will have similar sorts of differences from the R5 that the 6D series has had from the 5D series, presumably the R6 will have things like slower X-sync, longer minimum shutter time, and slower FPS (at least with tracking - I know rumour has the top line FPS being the same), one card slot and perhaps even an inferior EVF(?). The rumours peg the R6 at "just" 20 MP, and the latest information from CR Guy suggests the R6 will also have a plastic body (or am I reading too much into the statement that it won't be magnesium alloy?), will go without a top plate showing camera info and may be so small it works with the same add on grip the RP uses. To me, that sounds like a camera which sits below the 6D level, rather than at the 6D level. Also, it seems to leave a very large gap between the R6 and R5.
Perhaps I'm looking at it too much from my own perspective, but as a 6D and now 6D II owner, the thought of a camera with no top plate display, a small and plastic body, and 20 MP, doesn't sound exciting, even if it has some other features such as IBIS. And assuming the R5 has a 5D series price (if not more), I don't see myself buying it, and the existing R will be looking a bit "old" without IBIS and perhaps inferior sensor and EVF tech. A camera sitting in that gap between the R5 and R6 is probably what I'd be looking for if I was buying an R system camera, and about what I would expect for a 6D series price. (That said, whatever the specs of the R6, I could end up being surprised. When I read the specs for the 6D I thought I'd never be interested in one, and the same for the 24-70/4 IS and the 40 pancake ... and yet I bought and enjoyed all of them )