I really hope we get a 70-200/4 L which also has the extending design. The f4 model is a very nice half way house with top level build quality, optics and AF, a still quite wide aperture but much less weight than the 2.8. In the ef lenses the f4 was so long (though narrower and lighter) that it really offered little size advantage over the 2.8.
An RF 70-200/4L would be great for hiking, landscapes etc.
It actually wouldn't shock me that much if Canon did give an RF 70-200/2.8L that was non-extending. Canon has been doing lots of unprecedented things recently and moreover has always, always focused like a laser on professionals and the 70-200/2.8 as a key product. If professionals say they want the non-extending version then it would be very easy to do - the optical design could stay identical it just would sit in a different outer body. A fixed lens will likely be a bit more robust and better weather sealed no matter what - the RF appears excellent in these ways too but ultimately you can't beat physics. For travel and casual use the extending design looks great, indeed a revelation for a 70-200/2.8, but I could imagine pros preferring the fixed model.
It couldn't stay identical because the telescoping - or lack thereof - is factored into the optics. The only way they could do what you suggest would be to essentially wrap the existing lens in another lens body with gearing that manipulated the telescoping, while the outer body stayed fixed. That would be a wide lens indeed!