Sorry for the late reply. In the 2 weeks I've still kept using it and I learned a lot! But a lot of my musings still stand, at least in my mind, but after thinking a little longer, a lot of the thoughts I had also apply to other distros as well.
To answer what you asked in final, a good hypothetical that might answer it is something like GNOME. If the nixos channel blew up in a doomsday scenario, I'd be stuck maintaining my packages myself, right? And I use the doomsday scenario, because the problems here apply for self-made packages as well, but it's easier for me and maybe others to wrap their head around the problem I'm getting at. So with GNOME, I'd have to update every single dependency manually in my nix files. With something like Arch/Alpine I could just have those files, and they have these really neat scripts where I can just bump the version, and it'll download, set the hash up, and bump the version all for me. With Nix there are no such tools. I can't just automate the process, nor is it feasible to do this type of thing manually. As new features are added, so are new options needed to activate those features. And yes, although in this scenario, I would probably just opt to not add these options and set it up myself, when making a package for the general public this isn't the case. If GNOME adds a feature (idk why I picked GNOME I haven't used it in like 5 years) to have extensions managed by the package manager, I'd have to add an option for what extensions are needed and all that. And this is a lot of work, at least as far as I know. The extensions would also have to be packaged.
I'm aware, but the appimage I run (Slippi Launcher) will run other appimages, and appimage-run can't handle that, since it extracts the appimage, then runs the contents, but it won't automatically do that for other appimages that are run.
Which is why I used a Distrobox and it was awesome, worked like a charm. I used Arch previously, and I just made an Arch distrobox and it worked perfectly.