> So instead of comments with parsing directives, people use underscore prefixed keys to keep metadata and comments, that's not a win at all

Your comment doesn't make any sense. You're just pointing out that developers designed data interchange formats as subsets of JSON which explicitly support metadata and comments. This means they are specifying their schemas to support these usecases in a way clients can and do support it. That, if anything, proves right the decision to not support comments because it resulted in better-defined and well-behaving clients.