I believe that the opposite design decision has worse developer UX due to more footguns in unrestricted nix that may manifest during inputs outputs resolution

Additionally, all .nix files are still fully syntactically valid nix files, which is the pre-condition that most tools expect. It's just the fact that some flake.nix files are not accepted by nix flake tooling, even though they are syntactically valid nix. This is normal and expected. I do not expect a file containing 5 to be a valid flake.nix.

Now can you tell me again in plain words what you are complaining about "things should be the thing that they look like" is either fulfilled or much too vague

I believe that the opposite design decision has worse developer UX due to more footguns in unrestricted nix that may manifest during inputs outputs resolution

Additionally, all .nix files are still fully syntactically valid nix files, which is the pre-condition that most tools expect. It's just the fact that some flake.nix files are not accepted by nix flake tooling, even though they are syntactically valid nix. This is normal and expected. I do not expect a file containing 5 to be a valid flake.nix.

Now can you tell me again in plain words what you are complaining about.
"things should be the thing that they look like" is either fulfilled or much too vague

[elikoga@hpLaptop:~/Dev/thymis/why-are-we-having-this-discussion]$ nix eval flake.nix
warning: Path '/home/elikoga/Dev/thymis/why-are-we-having-this-discussion/flake.nix' should point at the directory containing the 'flake.nix' file, not the file itself. Pretending that you meant '/home/elikoga/Dev/thymis/why-are-we-having-this-discussion'
error:
       … while evaluating the file '/nix/store/ajcvhz83nc55rnrk0hlnhgi0cdi9mfbd-source/flake.nix':

       error: file '/nix/store/ajcvhz83nc55rnrk0hlnhgi0cdi9mfbd-source/flake.nix' must be an attribute set
thymis-controller-py3.13
[elikoga@hpLaptop:~/Dev/thymis/why-are-we-having-this-discussion]$ nix eval -f flake.nix
5

Which behaves exactly as expected

No this is a syntactic check. With flake.nix: import ./why.nix and why.nix: {} we get

elikoga@hpLaptop:~/Dev/thymis/why-are-we-having-this-discussion]$ nix eval flake.nix
warning: Path '/home/elikoga/Dev/thymis/why-are-we-having-this-discussion/flake.nix' should point at the directory containing the 'flake.nix' file, not the file itself. Pretending that you meant '/home/elikoga/Dev/thymis/why-are-we-having-this-discussion'
error:
       … while evaluating the file '/nix/store/q3vbzdgg0r1hdxki9zc2aqap78nl9qj8-source/flake.nix':

       error: file '/nix/store/q3vbzdgg0r1hdxki9zc2aqap78nl9qj8-source/flake.nix' must be an attribute set

Which tells us that this is not "just a typecheck on a value"