Earlier quoted context omitted.
The primary advantage of markdown is that it is more readable in plaintext.
I second this. I'm all in on Markdown and other ASCII-readable formats. By all means, render out to HTML to make things pretty, but consider keeping the source in plain text. You can browse my personal site with curl or even netcat: curl https://anderspitman.net/txt/19 nc txt.anderspitman.net 3838 <<< /txt/19
If your plain text document has line breaks to enforce some column width it will rarely match a browser's viewport width. This means the browser will break lines to fit in the viewport but still respect line breaks. So you get lines broken by the viewport and then line breaks a few words later. The text does not flow well at all.
If you only have line breaks at the end of paragraphs the text might flow better but the default viewport width on mobile browsers is (usually) 980px. Desktop browsers set their viewport width to the window width. In either case text ends up hard to read because lines end up really long.
You also lose hyperlinking which makes the web...a web.
If you want a plain text website just make it HTML with a minimal or no stylesheet. A single viewport meta tag will keep the page readable everywhere. Setting it to the device width will make mobile browsers reflow the text based on the screen size.
No one is going to read your blog with netcat or just curl into a pager.