Personally I have nothing against the Nix language, and use it without issue, but it's untrue to suggest that the language itself requires uncommon support for this kind of thing.
Terraform et al, despite not being my favorite, have much simpler semantics than Pulumi. It's not always a good idea to write DSLs into languages with huge paradigm mismatches.
> Why should I care about writing 'new' in front of all my declarative configuration?
Because that’s how your choice of language instantiates an object. Try F# or Swift or Go if it’s that annoying to you.
> What happens when an if statement depends on a concrete value?
What do you think “count = var.concrete_value ? 1 : 0” is doing in Terraform, exactly?
> The leakiness of the abstraction is too terrible to even consider.
While you are are entitled to your opinion, I’d suggest you are very much mistaken, and would implore you to actually consider it for a minute.