Go's runtime is thin: goroutines, a GC specialized for concurrency, networking, and little else. Java, by contrast, assumes a JVM plus massive stdlibs to handle everything from enterprise apps to big-data, making its platform genuinely "fat" and layered. Other Java-tier languages, C# included, follow the same model.
I agree Go's runtime is as thin as runtimes get. But having a runtime at all disqualifies it from being a C replacement. Rust is trying to replace C, Go is trying to replace Java and some of C++.
But then just stay truthful, otherwise C# is also a system PL.