[davidmanze]

I think the only aspect to keep in mind here is the simple fact that you can at least get decently fast 600mm zoom lenses for reasonable money.
If I had to choose between the Canon RF 800mm f/11 and the Sony 200-600mm f/5.6-6.3, I for one, would have a clear preference.
Of course, that's irrelevant within the Canon scope, but then Canon is a choice as well.

  If you are shooting Canon the lens road map is somewhat more limited, there are no 150/200-600mm zoom lenses at similar prices to the 800mm F11, the nearest is the 100-500mm which at the long end terminates at F7.1 and costs an arm and a leg ...... and no TPM glass either!
 So Canon have got you by the short and curlies ......

[Rover]

And now Nikon joins the 600mm game.
No, I'm totally not regretting skipping the RF system altogether, although there are some nice(-looking) lenses there. I don't owe it to Canon to follow their every zigzag though, even though I've been a user for 16 years (and counting).

[stoppingdown]

Same for me for Nikon.