So yes for me markdown is definitely powerful enough for blogging and complex technical writing - has been for the last 6 years- with a few small extensions and I’ll eat my hat before I use anything xml based or reinvent html…
If markdown were actually good enough, you wouldn't be reaching for bespoke extensions to markdown to make it more capable.
It looks like they've added support for some of the things I need (eg references). But not other stuff. It has hardcoded block support for Note and Warning. But it looks like I can't program my own?
It supports front matter, but only in a few predefined styles? And it looks like I can't define my own rendering / styling for image blocks? Like, if I want to make images clickable and be shown full screen, I can't do it using quarto?
Like I said in another comment, if you're writing a markdown-like document that can only be rendered properly in one bespoke tool, you're not writing in actual markdown any more. Like if I had my own C compiler with a bunch of custom extensions, code written which uses all of those extensions isn't really C code. With actual markdown, you can send people the markdown content itself and they can render it locally using whatever tool they like.
Use a tool like quarto if you want. But the need for something like this proves the point that markdown on its own isn't sufficient. If you're reaching for a markdown-incompatible document format, why stick with markdown at all, and not React, or ascii doctor, or typst?
I want figures. I want linked references. I want custom styling for images, and for blocks of text (eg warnings, notes, etc). I want a TOC and numbered chapters and sections. Sometimes I want a bibliography. Or a table generated from data within a JSON file.
You don't need this stuff for a readme file. But IMO markdown isn't powerful enough for blog posts, documentation or longer form content.