It would be very nice indeed to have an 800mm F8 to let in a bit more light, and I've argued here in favour of one, but it would probably cost at least twice as much, and wouldn't have the advantage of very light weight, which is what makes the F11 lens so popular.As for that 800m prime its an F11. If I where to take that to my local nature reserve my ISO would be very high even in daylight and I would have reduced subject isolation in my images which is not acceptable for the images I want to create and others will feel the same.
Regarding the 800mm F11, I've owned this lens for several months, and used it for bird photography in the UK, Gambia, Kenya and Peru, with very satisfactory results. It's great to be able to have a lens of this focal length that is truly easy to carry all day long - certainly far more comfortable than lugging my RF 100-500mm around! The stabilisation of the RF 800mm is extremely good, and more than offsets the disadvantage of a F11 lens, because it enables the use of slower shutter speeds.
F11 on an 800mm lens provides *plenty* of subject isolation too.
I find the lens is perfectly usable in overcast conditions, although *not* suitable for use at dawn or dusk. Typically, I'll be shooting at shutter speeds between 1/500 - 1/4000 depending on lighting conditions, and at ISO settings between 400-1600. At ISO 1600, I can still easily crop my R5 images down to 5120x2880 pixels, thereby entirely filling the screen of my 27" 5K Mac. I loathe noisy images, but with this set-up noise just isn't an issue, when using Topaz DeNoiseAI, or even the more basic noise reduction in Lightroom Classic.
It's clear from your post that you're condemning the lens without actually having used one. I'd seriously recommend that you consider getting it. While it won't of course match the sharpness, light-gathering abilities or AF speed of a RF 800mm F5.6, it will definitely enable you to get superb shots in anything but the poorest light.