Actually, I completely disagree. Rst can do things that markdown will never be able to do (at least, not any of the common dialects). E.g., you can create your own directives to do special formatting however you'd like. You also have a ton more functionality built-in (e.g., a directive for LaTeX-style math) that is also incredibly uncommon in markdown.

Furthermore, the document structure in rst is far more powerful (e.g., you choose your heading conventions).

As it turns out, I am not actually a huge fan of python (and there are plenty of problems with rst), but in the comparison between mdown and rst, the only argument I actually see in favor of mdown is that it's more commonly used (though the fact that commonmark has yet to completely take over actually mitigates that a fair bit).