The 200-600 isn't a professional lens and hasn't pushed Canon to compete with it. Nikon keeps pushing Canon. Sony hasn't made anything unique to their platform. Nikon has unique lenses you can't get anywhere else that push Canon to build unique lenses you can't get anywhere else, Sony doesn't have any unique selling point. Sony being mirrorless was their unique point and they have 'modern' marketing, but nothing in their system is the best at anything nor unique.
Compelling reasons to get into Canon: Large range of TS-E, fat 70-200 f/2.8, MPE-65, 200-400mm TC, no compromise f/1.2 primes.
Compelling reasons to get into Nikon: f/1.8 professional primes that cost bugger all, 300 and 500mm PF, 180-400 TC, range of PC/PC-E, no compromise f/1.2 primes.
Compelling reasons to get into Sony: Arguably better AF in their current bodies, open to third party lenses (that all make the same staple lenses).
Of course, this is my opinion, but it looks very much like Canon and Nikon are rivals and have been for decades. They both have lenses you can't get anywhere else. Sony doesn't have this. Why would I buy into Sony when Canon and Nikon have a better version of the lens Sony wants to sell?
I owned and loved the 200-600. There were some dumb things about it (the strap anchors weren't at the center of gravity, for instance), but it in fact was the best quality or image and speed of focus for a mid-aperture "white" zoom. Canon didn't have anything as good at the time in the range. That's coming from a guy who put more than 500,000 frames on a 100-400 Mark II. I now shoot the 100-500 on the R5, and I think the image quality is as good, but the range and aperture are worse in exchange for better ergs and size/weight. It's just a trade-off. If canon made a 200-600, I'm I'd likely prefer it over the 100-500 when I went out on hikes.