Pure rust should mean "only uses rustc" not "only uses cargo" imo. Cargo is great, but it is not the language itself or the only build system for rust in the world.
Cargo is great compared to other language build systems, but people tend to forgot that there are even better solutions like bazel and buck2.
You're not going to win this battle. The best you can do is to accept it as the canonical Rust project layout, and reimplement Cargo's build steps in the build system of your choice.