Don't want to be a troll, but if you are writing anything that is not a README and/or is a book or booklet or bookish, do yourself a favor and use Asciidoc instead.
This! Asciidoc is the grown-up brother of Markdown. Designed to scale up to entire books, handle images, tables, references, citation, book indexes, maths.
Absolutely. AsciiDoc (2002) is actually 2 years older than Markdown (2004), but is surprisingly similar to write.
It was created as an equivalent to DocBook XML for the creation of book-length technical documents. It has a rich history and is well supported in many places (try writing a README.adoc for your next GitHub-hosted repo).
It is also currently undergoing a standardization process:
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.
