The installation on my laptop is on its 4th hardware refresh. Rsync or brtfs send to a new box, modify the partition UUIDs, rebuild the initial ram disk, and you're on you're merry way.
But after about 5 years I would start having problems on my Arch installs and realize that some config I edited years ago is now causing a problem. I have to see if I should revert to the default config or merge in changes and if I need a new workaround. Overall the system was just accumulating cruft that ended up biting me. I never knew what configs I had edited and what the default was, experimenting was annoying forms of backing up the config, making changes and then figuring out what I actually changed.
NixOS is completely different because all of my "customizations" live in a Git repo. I have change history and it is trivial to remove things that aren't needed anymore and know I did a clean job. The concept of a re-install even doesn't even make sense. Every config change or update is just as clean as a re-install would be other than "state" like a database or my documents.
There are definitely a lot of differences between NixOS and arch such as many packages requiring patches to function at all or as expected. But "never reinstall" was a significant part of my ramp from Arch to NixOS.
Yeah, that's a fair point. You do get in the habit of occasionally diffing your config file against the ones with the .pacnew extension. As well as purging packages.
I've since migrated to a flake-based setup with machine-based variations (for my laptop and desktop), including easily swappable desktop environments. At a whim, I can switch between sway, hyprland, and gnome. This was mostly a result of me exploring/tweaking these without wanting to discard the configs; I always end up coming back to re-explore tiling WMs.
My experience through all of this has been great. I've even done a full re-install on both machines when the xz vulnerability was discovered and the process was effortless. That includes lanzaboote for SecureBoot, LUKS, and out-of-tree git-based flake builds for custom applications I build from source.
The one thing I found really helpful when starting with flakes was this repo that includes starter configs to help flatten that initial curve: https://github.com/Misterio77/nix-starter-configs/tree/main
Arch is especially nice because it has best in class documentation and vanilla packages. The installation is a bog standard chroot and use package manager like Gentoo had for ages or an old school Debian. Arch is nice because it’s as vanilla as a Linux distribution can be.
NixOS is the polar opposite of that: basically no documentation, weird behaviours everywhere, custom configuration, custom file system. Pretty much as far removed from Arch as something can be.