I think the argument comes down to complexity. Getting Markdown --> Pandoc featureset up to a reasonable fidelity is the next stage IMO, especially if Markdown can be made to support the most common downloaded TeX extension packages.
I think the best future option is going to be djot[0]. It is being created by the author of pandoc, who might possibly be the most qualified person in the world to appreciate all if the nuances of marking up text and parsing it.
The rationale page can do a better job than myself. https://github.com/jgm/djot#rationale
: echo "word**withboldtext**inside" | pandoc
<p>word<strong>withboldtext</strong>inside</p>
: echo "a*?*b" | pandoc
<p>a<em>?</em>b</p>(which is why markdown still doesn't officially have any maths support, only certain non-standard flavours do)
Feel free to come up with something new that supports Markdown as a subset or overlaps with Markdown.
Markdown as-is is lovely.