Out of curiosity, what do you know about electronics assembly lines? I admit, from what I know about them, this sounds like one of those ideas that's cool on the surface but impractical when you start digging into the details - even counterproductive. As I understand it, cameras are built on a single integrated assembly line; adding options would require either breaking up the line at every point there's an option choice - slowing down production and increasing costs for everything - or creating a second line just for custom builds, with all the expense that incurs. And since custom builds would be a separate custom process, they'd be more expensive than the standard build - possibly wiping out any savings from reduced parts costs.
Cameras are one of the most integrated bits of consumer electronics gear I can think of, short of a smartphone. Changing any part of that would cause knock-on effects for the rest of the system, if they were doable at all.
When I was studying at university (more yhsn 25 years ago hahaha), to save a little money, I worked on the assembly line at the Volkswagen factory that made the VW Polo model during a couple of summer holidays.
Each car manufactured was different in terms of engine type (diesel, GTI, etc.), gearbox type (automatic or manual), wheel type, interior trim (seats, tray, steering wheel, stereo, etc.), and this did not slow down the speed of the assembly line or final production. The logistics of the different pieces simply worked like clockwork.
I have never worked on a camera assembly line, I have only seen a couple of videos about the Leica factory in Portugal and the Sigma factory in Japan:
I guess it's harder to make cameras with different hardware features, but let's fantasize a little.
Let's imagine that you can configure a camera with three types of screen: folding, tilt or like the S1R ii.
Or you can choose the EVF and its resolution: 2.36M / 3.69M/ 5.70M or 9.44M dot. Or the sensor type and processor, because these two things usually go together.
It would be great if you could choose a stacked sensor and an ultra-fast processor if you're shooting sports or moving animals. But perhaps another photographer would prefer the 61MP sensor and a slower processor. Or for another, the dynamic range of a lower-resolution sensor, like the 12MP one in the Sony A7Siii, is more important.
I'm not saying that you can choose every single piece of a camera, but you can choose 3 or 4 relevant things.
Just like when you configure a laptop on demand.
To use your example: 8K video, from a hardware standpoint, would be a function of the processor, which is then programmed to do 8K video, and the cooling system which keeps the body cool while doing it. The processor is needed for other things like hi-rate burst shooting and handheld hi-res mode, so you can't drop that. And I'm not sure how the cooling system could be made optional - what do you do, leave empty spots in the body where the fans would go? - and I doubt you'd save significant amounts of money on parts by doing so - maybe $5-15 USD?
For example, I'd like my Lumix S5iix with the S1R ii's EVF and screen. Because I don't need the 44MP, 8K video, or internal ProRes RAW recording. In my case, I record podcast interviews that can last more than two hours without interruption. I don't know if recording in 8K would be feasible in terms of overheating and storage.
I think it's great that Lumix offers these options, but if you don't need them you might as well save them. And pay an intermediate price between the S5iix and the S1Rii. That's what I mean, although I understand that asking for this kind of flexibility from camera manufacturers is a bit naive on my part.