SGML is a meta-language unlike every other example in your reply, so the prevalence of such semi-structured languages (including SGML applications) doesn't just

SGML is a meta-language unlike every other example in your reply, so the prevalence of such semi-structured languages (including SGML applications) doesn't justify SGML itself. Even HTML is not exactly an SGML application (except for HTML 4), and in my knowledge implementing HTML with a generic SGML implementation was rarely done. So the fact that SGML is a near superset of both JSON and XML doesn't mean much.