Actually, a lack of license doesn't mean, "You're free to do whatever you want". Itt means "I retain full copyright and don't give anybody any permissions".
For me it's a problem for the exact reason you think it's fine: I don't want centralization. If I did, I'd go to reddit. I do want each topic of discussion to be spread out amongst different instances and communities. But for that to be viable, you need a way to get all the content as easily as if it was all in one place.
I think it can in theory, but there will be some problems. But most likely Silverblue or something else would have its own problems trying to implement something like that - I don't have any experience with them and don't know how they'd compare.
There are ways to secure the update process. For example, you can enable secure boot and store your secure boot keys encrypted (or on a smart card). Then (if a full chain of trust is implemented) to update your system, you'd need to enter the private key password (or insert the smart card), and a root-access executable couldn't to that automatically.
While technically possible, you wouldn't want to compile everything locally on NixOS. Only packages that you've made changes to (such as applied a patch) will be built locally, and everything else (by default) will be pulled from the precomputed binary cache.
You can disable the binary cache, or make changes to every package. The thing is, if you update a nix package, you'll have to rebuild everything that depends on it, and with lower-level components, that can be literally everything. It's not a sustainable workflow.
NixOS is not the most efficient distro either. I already mentioned some compiler optimizations are disabled by default, because they break build reproducibility. It also tends to use more disk space than other distros. So actually trying to super-optimize every package on it is somewhat pointless.
You could apply patches or change the build process. But there are some limitations to ensure reproducible builds. For example, compiler optimizations that break reproducibility are disabled.
I think you could disable build reproducibility to get rid of those limitations, but I haven't tried it.
/nix/store is immutable. But there are some files in other places like /etc and /var that are mutable. Also I (or a malicious executable) could, in theory, delete store symlinks and replace them with mutable files. Impermanence helps, but you'll still want some mutable state.
Fully immutable systems have everything outside of /home read-only. NixOS is not one of them.
To me, the smaller userbase is actually a real problem. I'm willing to stick it out and hope it grows. But for over half of the subreddits I subscribe to, the corresponding lemmy communities have 0 posts this last week.
Yes, I don't need 10k comments on my posts. But memes or mainstream news was never the big value of reddit for me - I can get these anywhere. Instead it is about the niche communities with a few thousand subscribers. And for now, I still have to use reddit for them.
It's a somewhat immutable distro, that is however fully configurable.
The configuration is all in one place. No more changing a bunch of files in /etc, some in /lib, etc, and having to remember all files you've changed.
You can easily recreate your system from your configuration or boot to older configuration.
You can easily open shells with different programs available. Very useful for development, when you need a reproducible environment with the project's specific dependencies.
Very hard to learn, but if you have learned it well, a lot of things become easier than in other distros.
It's not that users want to centralize everything. It's Lemmy's design that promotes it, because despite federation, there are still advantages to choosing big instances and communities.
Joining the largest instance makes searching, joining, or opening communities much more seamless.This can be addressed by:
Improving the search so that it can find communities, or even content, that no one on the instance has subscribed yet.
Making it easier to open a community in your home instance.
In addition to Sub/Local/All feed, you can have a "moderated" feed (with communities selected by admins). The "local" feed is most useful for instances on a specific topic. But for very small instances, it'll be too empty at least at first. So a moderated feed can create an on-topic feed that's more lively.
For most topics, only the largest communities are large enough to have good content, so everyone wants to join them. To address this, you need some easy mechanism to subscribe to all communities on a topic. For example, we can let communities follow other communities. Then people can create topical meta-communities that aggregate content without centralizing it.
AFAIK lemm.ee fits all your requirements, what don't you like about it?
Edit: Maybe I'm wrong. It's run by Estonians, but looking up its IP, it point to the US, so maybe it's not hosted in the EU.