the argumentation is, that you have access to any part of the system and can change everything. What is the reason that you cannot turn NixOS into Arch?

Because you don't have access to any part of the system. Again, nobody is saying you can change everything. NixOS is immutable meaning that is locked down read-only and only the Nix daemon has access to write. It's a matter of declarative vs imperative configuration schema. It's how vs what. In NixOS, you tell the system what you want done. In Arch, you tell the system how to do it.

So, you can't change any distro into any other distro, but the likelihood of two distros having any meaningful difference is low to the point of being pointless to ask.

As another user already stated, the main reason distros don't matter is analysis paralysis. Most of the users asking for thoughts between distros are effectively asking which of two duplicates is better. Say you've narrowed the distros you want to use to Debian and Kali, what is the difference between them if you just want to play games? There isn't one. You can run proton on Kali just as easy as Debian (you'd even be hitting the same repo likely).

Because you don’t have access to any part of the system. Again, nobody is saying you can change everything.

But you have! And as said, there are people saying exactly that. BTW I actually understand the difference between NixOS and Arch. I mean you can actually change anything on NixOS and slowly transform it into a regular distribution without what it makes to be a Nix system. I mean if you have root rights on the system, then you could remove what it makes a NixOS and install a different package manager, set it sources up from Arch repositories in example and so fort.

I don't know if everything can be done like that, but theoretically there is no limit in access to any file and data on a Linux distribution. If you have root access. In example, what in particular cannot be changed in NixOS?

Ok, this is getting into macro scale now. They use entirely different filesystems, boot sequences, package databases, and low-level C libraries. So, in that car analogy what you are describing would effectively be changing out 1 or a few parts at a time and iterating over that until no original part exists and then saying you turned a car into a truck because you kept the car radio the same.

Maybe this is a matter of perspective, but at that point I would say you didn't turn a car into a truck, you just built a truck.

EDIT:
I would say most people's constraint on being able / not able to convert distros is "within reason". Without that, we might as well just talk about kernel versions because that's ultimately what Linux is not the DEs and package managers and etc,etc.

To your original point that you can't turn one distro into another (which you seem to now disagree with), you can but not always "within reason".

Yes, that's the argument its being made.

I'm NOT (edit: forgot the NOT, lol) the one claiming that, just reiterating whats being said. Because I don't know and want to find it out with discussions. I think it is ridiculous, and don't think anyone should do that. But technically it can be done, seemingly.

I've been thinking more and more about this (the NixOS to Arch) and comments saying "that should be easy".

Dynamic linking creates a catch 22 to all of this.

You have to do the majority of steps live and can't reboot. NixOS doesn't follow FHS, but Arch bootstrapping requires that. If you force-create those directories and try to bootstrap Arch over a live NixOS instance, the binaries you compile will instantly break because they won't be able to find the dynamic linker (ld-linux) or standard C libraries (glibc) in the locations they expect.

At some point you are going to be using Nix's development tools to build out pacman. To get around the previous issues you would drop to a nix-shell to build the environment (which is one really good use of NixOS in general), but then you'd segfault as soon as you tried to use it outside the environment.

Even with the pacman binary present as soon as you rug pull glibc from memory, since pacman is needing the host's instance of that, you'd have a kernel panic.

I mostly agree with this. The reason I stick with arch has nothing to do with finding it technically superior or more convenient to use than other distros. There is almost no way I would be using arch if their wiki wasn't leaps and bounds more helpful than other distros' documentation. Some other distros can meet my needs just as well or better, but arch's documentation allows for a more straightforward learning process through tinkering.

I do think repository differences can potentially matter a lot. I have a lot of respect for hyperbola being ultra-hardline in removing proprietary packages and any hint of uncertainty in licensing, but the fact that that kills off texlive makes it untenable for me to use. Keeping manual installs up-to-date is a hassle, so I can definitely understand someone limiting themselves to distros supporting a certain set of packages.

There is another point, which makes this discussion very variable. Also the choice matters or does not matter a lot, depending on the person, expectations and what is being done. This is probably the biggest reason why we don't agree on simplifications like these. And BTW, just because the examples I gave are extreme does not make them wrong in any way. They are just easy for illustrating my points I'm making.

If someone is coming from Windows, does not care much about trust and just want something that runs a browser, doesn't care about community or technicalities, then yes it does not matter if the person chooses Ubuntu or Mint. On the other hand, if someone doesn't like corporations, has strong opinions about standards and is a developer, then the choice suddenly matters a lot more.

choosing a distro with specialised security hardening (immutable systems, Nix, Qubes, Bazzite) does matter; most of these will make new users unhappy or even question their sanity.

Out of curiosity, as someone who's never used Bazzite/other uBlue/SilverBlue/etc, what makes it difficult for new users? I definitely agree with Nix and Qubes though (and SecureBlue to some extent).