They save on the ARM licensing fees. I'm not sure about the details, but I believe it's both a flat fee and cost per chip produced
They can freely add proprietary vendor extensions, which I'm not sure ARM allows
They're less restricted in general chip design (I think ARM has restricted options in the last years, Apple has some special privileges as a founding member)
The thing is, it's all a grift, but there's only so much that can be grifted. Some of them were eating it up because they were hoping that once in charge, they'd benefit, but there's only so much space at the top
The average Linux user definitely will not care about reproducibility.
I think a lot of people do care about it, just not under that name. But I think a lot of users asked themselves at least once "what did I do back then to achieve X". Not in that the whole system is reproduced 1:1, but certain aspects. That's something much easier to answer with nix.
Well, you don't need to learn nix as a programming language for a simple installation, you can use it like a slightly different json, which the configuration.nix part was about. You can get the reproducibility aspect from just that, so I wouldn't say you get no benefits at all without learning the language.
There are more disadvantages (like time required to rebuild because you added a single package), so Arch is the better choice depending on preferences. Arch is a very good traditional distribution in my opinion, can't go wrong with it
Arch is easier in my opinion, at least if you want to leverage the power NixOS can offer. A simple /etc/nixos/configuration.nix maybe not, but once you enter custom options / submodule territory and use stuff like lib.mapAttrs, I'd say NixOS is quite harder. Or just a more complex overrideAttrs. But then again, Arch doesn't have an equivalent to that...
Like I'm not getting a dedicated router and have no server room in my apartment, and my consumer router only supports two VLANs (main and guest). But I'd say the rest is rather sophisticated with all machines defined in my NixOS config, including automated generation of firewall and reverse proxy rules for which I wrote custom modules.
Media server isn't super full but connected to jellyseer and the rest of the stack, accessible over TLS (Let's Encrypt certificates) only, with the option to have users managed via IDM.
However, I only have devices on my network that I somewhat trust, with an Android TV box being the worst offender. The smart TV was never connected to my network.
Would be cool to isolate my work PCs somewhat (I work from home with company provided equipment) but it's just not worth the trouble in my opinion. Not switching out a low power device that does most for two different devices that both use more power (since you usually need a router and a modem).
If you are particularly deranged, you can also use "^^"