The wimpiness of the Nix language is a virtue. There is no call/cc. There's no ports, and indeed networking is tamed. The language is just barely powerful enough to do its job, and community members have found it somewhere between intractable and impossible to build compilers, text processors, etc. in Nix itself.
String interpolation is a fitting punishment for our decision to continue to use Unix-styled systems; our systems think in bytes and communicate in bytes, and I see no reason why we should walk away from our highly-developed long-standing relationship with bytestrings as long as we are still on Unix. (You will say something beautiful about Scheme. I am busy looking ahead to capability-aware languages; Nix is a scaffold and nothing else.)