AsciiDoc is so close to being good. It slam dunks Markdown, but they just have a few nagging issues that they refuse to fix, for 9 years now:
I think this is actually a great example of how dedicated Dan Allen is to the project! He has been following up on the ticket for over 6 years!
I have interacted with him briefly on the Antora Zulip Chat a couple of times and he is always very helpful, it's a pleasure.
I am not sure what the worry is with backward compat. The only difference with the resultant HTML, is extraneous "p" elements would be removed from "li" elements. Anyone who has been relying on that behavior, has been relying on non-idiomatic HTML and should be inconvenienced.