Respectfully, I’ve never forgotten a call site, but also yes. In a hypothetical HelloWorld service, the HelloRequest and HelloResponse generally aren’t used anywhere except a rpc caller and rpc handler, so it’s not hard to “remember” and find the usage.

Some callers may not need to update right away, or don’t need the new feature at all, and breaking the existing callers compilation is bad. If your caller is a different team, for example, and their CICD breaks because you added a field, that’s bad. Each place it’s used, you should think about how it’ll be handled, BUT ALSO, your system explicitly should gracefully handle the case where it’s not uniformly present. It’s an explicit goal of protos to support the use case where heterogeneous schema versions are used over the wire.

If a bug is introduced because the caller and handler use different versions, the compiler wasn’t going to save you anyways. That bug would have shown up when you deploy or update the client and server anyways - unless you atomically update both at once. You generally cannot guarantee that a client won’t use an outdated version of the schema, and if things break because of that, you didn’t guard it correctly. That’s a business logic failure not a compilation failure.