> Go is seeing adoption that no other “new” language has exactly because of its simplicity

Yes - for me, the simplicity is essential. As a part-time programmer, I don't have months to spend learning C++ or Rust.

If my project needs to compile to small(-ish) standalone binaries for multiple platforms (ruling out Python, Ruby, Java, C#, etc) what simple alternative language is there? Plain C?

C# can compile standalone binaries for multiple platforms.

Basic Rust doesn’t take months to learn, especially when you’re not trying to do things like distributing crates to other people. I found the compiler to be enough more helpful than Go’s to make them roughly time-equivalent for the subset of common features, especially for a simple CLI tool.