I have a 5DSR and a bunch of lenses, and don’t get me wrong, it is a fantastic camera...but, innovation in the form factor of cameras would be a welcome change. The existing industrial design is overly informed by the constraints of a roll of film, but, is not either the most ergonomic or the most stable position to hold a camera. We could do much better if we’d give it up.
Small size mirrorless bodies really show what’s wrong with the current style, too. They’re difficult to hold on to, and the buttons are virtually all dual purpose. At least the 5D has a nice grip, but with any kind of telephoto it becomes pretty awkward to stabilize.
Finally, with a “pistol format” camera, you can extend the viewfinder back from the lens, which can let you keep the existing big lenses and sensors but make an overall smaller package (the components move into the grip). This shallow sensor-lens depth is the main impediment to putting full frame glass and sensors into the standard small size mirrorless bodies. As has been cited elsewhere in this thread, smaller glass with higher density sensors is not a long term solution. Big glass does have a lot of advantages for high end photographs.