Why dismiss the RF 100-500 because it's F7.1 at 500mm? You say that the Nikon 100-400 is great. That's F5.6 at 400mm. That, of course, is only an approximation. Variable aperture zoom lenses actually vary continuously as they zoom. The RF 100-500 shows F5.6 at 400mm when the camera is set to 1/2 stop increments, and F6.3 at 400 when set to 1/3 stop increments. Probably, the actual aperture at 400mm is about F6. For all we know, the Nikon could be F5.9 at 400. In any case, the Canon's maximum aperture at 400mm is no more than, and probably less than, 1/3 stop slower than the Nikon at 400mm. If the Nikon is great, it's hard to see how the Canon could be disqualified by at most 1/3 stop slower aperture at the same focal length. And, you get an extra 100mm with the Canon. You don't have to use it, if you don't want to, but I can't see why you'd reject the lens for actually offering more. And, by all accounts, and judging by numerous samples, the optical quality of the RF 100-500 is top notch, at least as good as the Nikon 100-400.
But, not only does Canon offer what is at least a match for the Nikon 100-400, at a similar price, but they also have a unique lens in the RF 100-400. It's between 2/3 and one stop slower than other 100-400 lenses, but it's so much lighter, smaller, and cheaper. It's a sheer joy to use. In my opinion, that lens alone is a reason to switch from either Nikon or Sony to Canon. My view is that Canon's RF lens lineup is actually far better and more interesting than what is available from other manufacturers (there are also gems like the 70-200 F2.8, 16 F2.8, 85 F2 IS, 600 and 800 F11, etc). For me, it's not about sheer number of lenses available, but what those lenses are. I have been delighted with every RF lens I've bought so far (I currently own nine of them and the 1.4X extender).