Changing NIP-54 now to use asciidoc instead of Markdown could likely cause confusion and break anything. I think no one wants to rewrite everything in asciidoc. If anything, a new NIP for asciidoc would make more sense if you ask me.
Discussion
There is one good idea in the comments for this pull. Instead of scrapping Markdown completely, define a separate kind within NIP-54 for Asciidoc wiki pages. That way it wouldn't break anything.