Haven’t we more or less done that? There will of course always be Markdown projects that don’t use CommonMark and that won’t change. But every note taking app and website I’ve that supports Markdown uses CommonMark.
The only problem is that you can use extension in CommonMark. So although GitHub and my note taking app both use CommonMark, they might not support exactly the same features.
God no. CommonMark is where Markdown goes to die by never changing and promoting the most basic and many times worse implementation. Markdown should evolve to enable new features and improve, not fall victim to people wishing to prevent its advancement.