[Brightcolours]

1:
First, Canon already has 600mm f4 and 800mm f5.6 lenses to mount on any EOS R camera, it is not as if RF mount lenses like that will be smaller or lighter, just because of the mount differences.

2:
If this rumoured 800mm f11 is inexpensive and comparatively lightweight, it does make sense. The loss of sharpness due to diffraction can be mostly compensated by Canon's DPP software with the appropriate lens profile.
How a 600mm f11 prime from Canon will have to compete with 3rd party 150-600mm zooms, not sure. We will see which lenses actually will materialise.

3:
And the donut bokeh of mirror lenses is due to the mirrors, not due to the small f-number ;-), Dave. And how often do you do your birder thing when it is not bright and sunny? ;-)

  3:  I think I know mirror- lens doughnut bokeh is down to the mirrors BC .. really !!
...... the inference was "is it a mirror-lens?"  (tongue in cheek joke ha ha!!) ....... which of course it won't be .....

 I go birding when it's sunny mainly yes for the pop !!...... and as I said the 600/800mm F11 will only usable in sunny weather (for anything that moves) .......
....... but the G2/Sony/Sport is 1 2/3rds stops brighter at 600mm ..... and at 150mm it's 2 1/3rd stops brighter ...... that's huge BC !!  .... and the difference between 600mm and 800mm isn't !! .. 
 Those who chose the Sony 200-600mm will be getting the best deal as the their 1.4X TC hardly drops the IQ and you get a 840mm F7.1 lens.

 How can you argue with that ??

How can you argue with what, exactly? You make little sense.

I wrote "How a 600mm f11 prime from Canon will have to compete with 3rd party 150-600mm zooms, not sure."

So are you making up arguments, or what?
And at 150mm? Are you just writing nonsense?  Or did you by accident "reply" to my post and started to argue with yourself?

So you appear to agree with me that you "bird" in bright, sunny conditions, and that 'so only " usable"  in bright sunny conditions.' (what you wrote) seems like an odd thing to complain about with birding in mind. Of course, you do get much better subject isolation with 600mm f4, and Canon offers such a lens already, like your Nikkor.

It appears that these 600mm and 800mm f11 lenses will bring something others do not yet offer, small and compact tele prime. If that will be attractive for certain uses/users depends especially on price, weight and size.

It is said that these 600mm and 800mm lenses will be DO lenses.

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Yeah, f11 is too slow really.
It should at most be f8 and preferably f6.3-7.1.
Otherwise, why would one get one of these instead of an MFT system for instance?

Just add to the list stabilisation ..... at those long FLs IBIS has a limited effect .... maybe it will need VR as there's no headroom for ultra high shutter speeds.
  

Canon lenses never have "VR". Since the rumours call the lenses "600/800mm f11 IS DO STM",  it is of course apparent that the lenses have IS, and the coming EOS R5 is said to have IBIS which works extremely well in combination with IS. "The EOS R5 will be the first Canon camera equipped with IBIS (In Body Image Stabilization) and when used in conjunction with the extremely effective in-lens stabilization (IS), will allow photographers to handhold the camera in light levels not previously imagined. "

And thxbb12, that is a bit of an iffy "point" you try to make, there. Just because Canon is (rumoured to) going to announce a  cheap and slow tele, does not mean you have to use only that one lens on an EOS R camera. You can take advantage of a faster/more shallow DOF capability in for instance the portrait range AND also choose to want  super telephoto ability which is relatively easy to take along and affordable, at the same time. 
I can relate to that: I chose a Canon EOS 6D for its FF ability to easily have shallow DOF, with for instance my 55mm f1.2 and f3.5 lenses, my 70-200mm f4 and 140mm f1.8 lenses. I also have a Tamron 500mm f8 lens, which takes as much space in my backpack as my 135mm f2.8 lens, and is very lightweight. So I can have 55mm f1.2 "standard prime" and 500mm tele at the same time, in a small camera bag or backpack.

I can also see advantage for others in MFT, of course. Not everybody is the same and has the same needs.

If these rumours have things right, it appears that Canon is aiming for small and light options with these two lenses. What the rumours say is that they are NOT "L" lenses, that they use the cheaper STM instead of Nano USM focussing, that they will offer IS and use DO optics to curb their length and limit to f11 to curb their weight and price.

For fast tele, the whole EF range already is available for EOS R, and apparently Canon chooses to first come with slow and cheerful super teles before coming with the big and heavy great whites in RF guise.