The learning curve for RST is generally higher because it does more. You'll notice that Markdown's learning curve goes up quite a bit as they add all the features that RST has. That said, RST has some bad design decisions, but I'd say about 75% of the complexity it introduces is needed, and the 25% is bad design.
If you have to extend Markdown, the battle is lost. The basic core idea, is OK, but as soon as you find yourself needing to do more documenty things (lists with formatting, tables, etc.) its the wild, wild west - different extension standards CommonMark, Github style, etc., editor, or library of choice.
Not to mention all the special-custom-component standards now which require code... the whole point of Markdown was you could just use any old text editor and be just as well off as looking at a prettified version.
You have the higher complexity and learning curve anyways, more, even.
I guess, the bad design decisions in RST are bad, but there are equally bad design decisions in all these new extensions, editors.
Instead of trying use a hammer put in screws, we should just change tooling. If you need document-level formatting etc., plain-text formatting is cool but don't try to be markdown. Use markdown when you just need simple text docs, if you find yourself doing complex stuff maybe you shouldn't be using markdown, or at least not calling yourself "markdown".
