I have honestly avoided NixOS for quite a while because the situation seemed "unstable". I don't particularly care for the complaints brought about by the open letter, I do however worry if a distribution/community is imploding.
Regarding the stability of the project: The project and distribution is incredibly vibrant, and not going anywhere. There are tens of thousands of organizations that depend on it every day, and projects that are funded on the time scale of decades. It's here to stay.
1. Reducing the number of moving parts, so there are fewer interfaces that need to be documented and understood ... because they don't exist.
2. Resources like https://zero-to-nix.com/, and https://docs.determinate.systems/.
This is a big lift, though, so anything we do here in the near future will definitely not be enough.
Zero-to-Nix looks better than the documentation I used to get started, seems a great resource. But even with the worse documentation I found, I was able to overcome the pretty steep initial learning curve. What did it for me was the lack of documentation for many packages and having to dive into the code every time I needed to configure anything remotely advanced. Whereas in Arch there's always a couple examples that get me 95% of where I want to go, making it very easy to figure out the rest.
But thanks for the effort, I'll be looking forward to giving Nix another go in one or two years to see the progress and maybe switch again, as I really liked the whole concept.
I like Nix, but what turns me off most is this kind of hyperbole from its evangelists. "tens of thousands of organizations" depend on Nix every day? How do you even come up with that kind of number? And as someone who has actually worked on projects that are funded for decades: the first rule is that things are changing, so stay flexible, chose boring technology, and always have a plan for migration. Binding yourself to a niche framework that is also strongly opinionated with an arcane syntax is pretty much the worst you could do.
As you say, Nix is already over 20 years old, so yes, it's probably not going anywhere, but that goes both ways: it will probably not vanish in the near future, but it'll probably also not become really popular. And that's fine.
Nix feels similar, but for build systems, package management, and configuration management all at once.