I think you are mixing two things up. Some 4k remasters have incredibly distracting film grain. Blade Runner comes to mind. It's a matter of personal taste and how accurate you want the film to reflect the film source.

The more compressed the file is, the les accurate it is, and film grain is the first thing to get crushed by compression algorithms

This could just come down to the remux group using differently sourced BluRays. Not all BluRays are created equally, sometimes a US release is the best, other times it might be an ITA release, really depends. A good remux group looks at all the sources and chooses the best, sometimes they’ll combine different parts from multiple sources as well.

In my opinion, the release with the noise in the comparison shots is the better release from the details retained.

I’m no expert, neither do I know what releases you downloaded and if they are quality groups. Just from the picture I’d say it’s to be expected because compression removes detail and detail is what you experience as noise in the picture. If I watched on my monitor compared to my tuned in TV I might perceive more detail as noise as well

Remux all the way. If the movie is grainy that’s just how it is. I understand some people like smoothing but I’d rather the full quality.

If there is grain in the source original, then a "cleaned" version is not an archival copy in my opinion. Grain is difficult to compress as it is random noise, so the bitrates required to maintain it are high.

If you see grain in a movie then it is meant to be there.

Film CAN look crystal clear and modern, but it also has flaws that show depending on how it is used and what film is used in the first place. if you start with high ISO film and push it in development it will have serious grain, but low ISO film fully exposed and well developed can be as smooth as glass, even on a 4K scan, problem is you have to have a huge amount of light to get film to look that clear and sometimes that isn't possible. you can get around some of this with day-for-night methods (filming fully lit then darkening to look like night) but that is a choice the director/DOP make in a production.

Digital cameras have grain too, and it can be much uglier than film grain but our modern image sensors can handle low light better than film (in some ways, it's a debate still) but even clear, well lit, well exposed, low ISO digital video productions will have grain added to tie scenes together visually if lighting changes slightly, to mask flaws, and to tie composites together (mixing cg and video for example). Small amounts of grain can actually enhance apparent detail as your brain fills in what it perceives as detail, Sony TVs can do this as part of their upscaling if you enable it.