I'm personally a huge fan of using MD for my stuff. I'm a recent convert, but am finding it works brilliantly for my use case which is idea capture, presentations, etc. Obsidian is my tool of choice. I've written a parser in php which converts this markdown into html ready for web publishing. All good.
But.
I also work with 50+ clients who want to manage their own websites. And they want to manage their websites easily, effectively and visually. They want things to look beautiful. They have audiences that they need to engage with content that is not just words but images, video, timelines, maps, iiif manifests, audio, sliders and much more.
This is a different audience. To ask my clients to "just boot up a jekyll instance and commit your content to GitHub" is the most wildly hilarious thing to expect of an editorially focused non-technical set of individuals.
Yes, plain text works beautifully for simple blogs, documentation, notes and more. But there is huge value in thinking about audience, about design, about engagement which goes beyond just a simple "text is right, wordpress is wrong" argument.
Then what seems to happen in geek circles is that all that "wonderful simplicity" is drowned by an absolutely insane set of tools to actually publish. Yes, you've got plain text files but if your stack then has some cli/npm/jamstack/js/gulp/brew horror attached to it, with a truly questionable maintainability problem attached to each and every one of those tools, how simple now?
Really the point is: different strokes for different folks. You can't make a stunning coffee table book in plain text. Much as you might hate it, InDesign is your tool of choice. And that's ok.
(Also, he's just plain wrong about not being able to get his content out of wordpress. There's a solid and very well supported XML export format for all wordpress instances, even wordpress.com...)