Earlier quoted context omitted.
That's a nice thought, but it comes with its own problems. For example, XMPP uses a sane subset of XML, which is nice, except that people throw full XML parsers at it (because why wouldn't you? Your XML library parses XML and limiting that is more work for the developer to do) and then end up with vulnerabilities they don't know about like entity expansion DOS's or system directive stuff (and YAML has lots of tricky…
We can agree to generate a subset - the things we won't add to a certain stream - even if we are more lax in what we accept by using full parsers.
We can agree, but that doesn't mean that others won't do it anyways; unless you operate in a silo, it's not likely that you're the only one writing software to use your system.
Standards get written and implemented by lots of people, and even tooling like Docker gets alternative implementations.