It's hard for me to see the 600mm TC as "master of none." It's sharp, versatile, and not all that heavy. As far as I can see its only drawback is that it is not consistently handholdable. For birds, especially, the advantage of 600mm over 400mm is HUGE. In fact, I cannot see why a bird photographer would choose the 400mm f2.8 over the 600mm f4, period. If I owned the 400mm TC I would be using the TC 90% of the time, which means it's the wrong lens for me.
The drawback to the 600mm TC is its absurdly high price, period. It's the lens I covet the most but don't own because my wife would hit the proverbial roof were I to buy one. It's sad that I could sell virtually
I concur with this. The RF 100-500 is a stellar lens. It's the only thing from Canon that I miss. Such a versatile wildlife, landscape, quasi-macro lens. and that extra 100mm makes a bigger difference than I'd ever imagined, over say the Z 100-400.
I really did enjoy the RF 800 F11 as well though. It makes a great "documenting" lens. In good light you can get good pictures, but most of the time I found I used it just as proof of life due to f11.
I agree, it's hard for me to say it as well. I just find that Nikon did such a good job with the 600/800 PF, that it diminishes the value of the 600TC. It should be seen less as a dig to the 600TC, and more as a nod to the outstanding PF lenses that have been produced.
You have 3 lenses that are all very similar and produce similar results (600TC, 600PF, 800PF). I could see the argument for the 600TC over the 600PF because it's 1.33 stops of light. But between the 600TC and 800PF, it's the 800PF almost every time. the 1/3 stop of light is negligible, but the 37% weight reduction and $9800 savings are not.
As for the 400TC vs 600TC, the 400TC can act like the 600TC - but not the other way around. The 400 + 1.4x + 1.4x is very similar to the 600 + 1.4x. So the 400 can double as a 560 f4 and 784 f5.6 as needed.
I heard somewhere that the framing difference between the 400/600 is 7'. meaning if you could get 7' closer to your subject, the 400 would have the same reach as the 600. no idea if it's true or not, but it seems to be the case. as I've gotten better at approaching my subjects, I've felt less need for further focal lengths.
I think it's important to distinguish what birds you are shooting. The majority of my birds are larger - so the 400 f2.8 is perfect. I use it at 400 about 80% of the time, and 560 about 20% of the time.
As a counterpoint to the bird thing, I have several friends who specialize in small birds (warblers, hummingbirds, etc.) and still use the 400TC as their main driver. if you can close the gap on your subjects, it works well. even if you cannot - you then have opportunity for environmental portraits.
To your last points, it depends in what environment you shoot. In a city, small birds (warblers) are quite used to people and you can shoot 400mm quite a lot and fill the frame. We have spots where kestrels are very close, but out in the country, you can't get near them so 600-800 is much more appropriate. I again agree, you can definitely get close to hummingbirds in many cases.