I assume the person you're replying to was referring to Markdown rather than Bookdown. It seems that Asciidoc was designed for the direction which Markdown has going with all these variants. If you find yourself chasing these Markdown variants to get more flexibility, then Asciidoc might be what you're looking for.

I don't know anything about Bookdown and it may be similar to Asciidoc. I would be willing to bet that Asciidoc would be around longer than most of the MD variants though.

Why might you want to continue using MD flavors? You already have loads of MD docs and you have no control over the processes which create them (shared environment, higher-ups force you to use MD, etc.)

NOTE: If you're interested in Asciidoc, also take a look at Asciidoctor.