People are free to knock themselves out with Bazel if they’re into that kind of masochism, but having it as the ONLY way to build your OSS project is a big no.

The problem with "the language tooling is already a build system" is that cross-language dependency chains are a thing. The moment you need a Rust or Zig file to be regenerated and recompiled when a JSON schema or .proto file is updated, you're outside what most of those language-specific toolchains can support. This is where Bazel absolutely shines.