I'm not sure I see how self-describing is a lower barrier to entry. It seems like generally the main reason why you'd want a self-describing format is if you're writing client (and server) language bindings by hand. If you have a tool to auto-generate those language bindings, you can skip that step, and there's no need to make a step easier if it's done for you automatically.

I can see where self-describing is better when it comes to side issues like debugging, exploration, or the hassle of configuring your build to generate code from the IDL files, though. But if I had to choose only one, those are lower priority to me.

I was under the impression it was trivial to store the message description in the message itself with protobuf.

But sadly it also means redundancy with every message sent, which must carrying this self-description.