Interesting (and long) thread. I only read the first page, but that's enough to see the OP's motivations are that he doesn't personally need the Z9's resolution or a lot of its other features and '... the motive here is that trading in the Z9 for the Z6iii will increase the odds I'll carry the camera and use it.'
Personally, I came from a 24mp Canon M camera to a 40mp Fuji. I love the ability to really crop to quite a small image and still have enough of a picture left to play with.
For me, the only problem with a 20Mp camera, like my Canon R6, is that I need to pair it with a lens of the optimum focal length for the shot. So often that is the one I left at home for the day!
Alan’s observation about cropping is even truer with the 60Mp sensor of the Leica Q3...
I had an occasion where I cropped a picture from my Z7 to get this almost usable picture of a frog in full mating call mode. It was shot with my 24-200. It really needed a 600mm lens
The important metrics are how many photons are captured by area and how much noise there is per area as opposed to your pixel centric metrics above.
I think you will find no advantage to smaller pixels using per area measurements. Particularly if noise reduction is applied before resampling the images to a common size.
Sensor technology also plays a role. Sony a9iii is an example of a camera with large pixels and low photon capacity.
I haven't seen any posts from Danno that contravene the terms and conditions of service. If we start banning people because some other people don't like their posts we open up a whole can of worms. Maybe at some time you'll make some posts som other people don't like, and then they'll start campaigning to ban you? For myself, I find the idea of posting on a forum suggesting other people are banned to be pretty toxic behaviour. I wouldn't suggest that you be banned for it, but I would ask you to stop.
The big advantage of the Z6 is size. The Z8 is a chunky camera. The Z6III is for most purposes as fast as the Z8, if you don't want the 45MP and wand a smaller form factor (and to save $1500) the Z6 III is the way to go.
I know, that is why I use monochrome sensors for most of my professional work
Even if the per area read noise is a bit higher with smaller pixels, detail can be traded for noise and you will still be better off with smaller pixels
In the end what matters is how much detail and noise is present in the developed image in standardized viewing conditions. Smaller pixels allow better noise reduction giving more detail with less noise even with higher read noise. As demonstrated by Jim Kasson here: Noise reduction with nonlinear tools and downsampling
Nope, upsampling does not produce anything else than a guess. But otherwise I agree: you don't have to downsample high res images to the lowest common denominator. You might as well try to upsample low res images to discover how inferior they are.
But upsizing involves interpolation which is much less of a guess than in any data manipulation involving extrapolation.
In my experience with today's modern AI driven upsizing any guessing is just about undetectable, especially under normal viewing conditions provided the upsizing is done within the limitations of the AI.
To compare image quality paired images should be analyzed at the same size. Resampling of one or both images is thus required if they were produced with different numbers of pixels. Smaller pixels provide more real data about the image projected by the lens, how you decide to use that additional information is obviously up to you, but I see no good reason not to trade resolution for noise if the end result is more detailed with less noise at the common image size.
As an example, I downloaded a1 and a9ii ISO25800 studio files from dpreview, examples where I expect read noise to be significant. They are supposed to have received the same exposure. I imported both in LR, applied luminance NR on the a1 file (20) before exporting a 6000px wide jpeg. The a9ii file was left to Adobe default and exported as a jpeg.
Here are a few screen dumps, a1 to the right.
I think the a1 file has more detail and less noise.
I think most lenses can produce aliasing in all current systems.
You missed the point.
I traded detail for noise taking advantage of the higher resolution of the a1. The result when downsampled to a9ii dimension showed more detail with less noise as predicted by Jim Kasson.
No amount of noise reduction on the a9ii file will bring more detail.
But it could be interesting if you make a test of your own liking showing the benefit of large pixels.