The main noise increase will come from loses from adding more pixels. Which has improved immensely over time. But I think the difference is not as pronounced as you might think, certainly not one stop in a fair comparison.

What you are seeing is just magnifying similar noise levels a lot more. But not because there is that much loss in adding the extra pixels.

The right way to compare say the R6 and the R5 would be to down res the R5 to R6 resolution. Then you will see the difference just in loses associated with adding extra pixels and not just zooming into the already existing noise.

Alternatively you can also upres the R6 to R5 resolutions to compare detail that way.

And what I mean by many forms of photography not needing high ISO's. I do a lot of portraits in natural outdoor light with primes, so I am always at ISO 100, ISO 200 if I am using Highlight Tone Priority.

If I'm doing landscapes I am not ussualy using high ISO either. 400 except in unusual circumstances.

If I'm doing birding, which I have not done in a while. Want to get into sports and I have done some events. Then yeah, I'm cranking those ISO's quite a bit. So clean files gonna be import for sure for stuff like that.

But with modern cameras being so good, we can raise those levels more and more every generation. I'm still using the 1Ds III and 1D III with good success, especially for the money. Definitely not as good for events and sports that I want to do. But I think it iliatrates a point that some don't need super high ISO's and some don't need super high resolution. And I think we agree on this.