Really? I was under the impression that a good quality 35mm film can resolve to as much as 8K without much trouble. Why would extracting 4K out of it be a problem?

It's only about 6K best case scenario and usually a little under 4K in practical resolution. Of course some of the major 6K or 8K cameras on the market are only a little better than half that in practical resolution. There's also a lot more to final image quality than just raw pixels or grain, it's just a small piece of the puzzle and why a 3.2K camera may look substantially better than a 6K alternative.

> Consider for a moment that 4k resolution is not easy to obtain from film

Why's it not easy? You just do a 4k scan of the film don't you?

Indeed and there's many methods for this that are being used to restore many old films from archive. Modern film cameras used by major networks record at 4K-6K (full open). Beyond that even if the cameras on set are recording at 3.2K or 2K pro-res etc... it doesn't mean that the post-production and up-scaling upon rendering doesn't benefit from a final output of 4K. Finally - we must think to the future and not hold back technology to the lowest common denominator.

VFX are substantially more expensive to complete at higher resolutions. Even in major films released at 4K, fx are frequently lower resolutions because of time and budget constraints. Going back and rescanning older films that have lots of cgi usually results in very ugly vfx scenes.

And again, resolution is just a small component of the final image quality. There is a major 3.2(ish)K camera on the market that has substantially better IQ than 4 and 6K alternatives. And another 6K camera that has substantially better IQ than 8K options.

As for upscaling, we're probably better off leaving that for the end user as better upscaling algos are created and deployed all the time and baking it in from the studio locks us into what will soon be old methodology.