Don't get me wrong, from what I hear Nix actually does deliver on the promise for the most part, it's just that you have to learn a new language to use it effectively and of course it has its own quirks.
Im not comparing whether Dockerfiles or buildpacks or nix packages are more ergonomic than one another but i do think your comment is...misguided. From what I have heard Nix is pretty wonderful to use and simplifies the problem - it just requires you learn about Nix a bit which i think is a fair trade-off for the benefits it supposedly provides
Nix ... I have so far spent about 10 hours learning it to manage my machine. I have forgotten about 98% of it and abandoned the project. You feel like you're sitting in the middle of a spider web, and you can sense the whole system at once. Literally none of your prior knowledge of how to use a computer will help you. None of your existing build tool CLI can be used. Every package manager needs a nix-ifier, like node2nix. Everything you see in a nix file will have to be googled, searched in the documentation, searched in GitHub repos for some kind of example. Nix has rebuilt the world from scratch.
If you're trying to make the next big thing, try to make it leverage people's existing knowledge. One truly excellent example is `compile_commands.json`. It does a very similar thing to Docker, where it extracts information from your existing build process, without actually changing the build process. The problem statement was that people wanted LSP (and predecessors) implementations to have access to a list of input files to a C/C++ compiler, but they didn't want to abandon Make and CMake etc. So they basically made a structured log of all the CC invocations, and a wrapper around CC that would parse the arguments and write to the log in JSON format. These days you get it for free with CMake[0]. You can use it with nearly every C/C++ build system on earth with a single CC=... argument to make.
[0]: https://cmake.org/cmake/help/latest/variable/CMAKE_EXPORT_CO...
To use an example from another community, no amount of performance improvements to NPM will ever make it a good idea to depend on hundreds of one-liner "is number odd" or "left pad" packages. Papering over the problem with yet more technology only ossifies it, making it harder to solve for real.
As you suggest, these are probably 'pieces' of the puzzle, by no means 100% identical to how containers are used today. But I think we'd have ended up in a different place.
There's definitely advantages that way, but there's also drawbacks.