The web won out over FTP and Gopher because documents could link to others for reference. Those systems could only link through menus (if at all).
HTML is actually pretty awesome. If you want minimal HTML just make minimum HTML. You don't need JavaScript or CSS. You can include a single meta tag with a viewport directive and get a magically responsive page perfectly readable between desktop and mobile without having hard line breaks.
Plain text on the web sucks. If the browser gets a header of "Content-Type: text/plain" what's the encoding? Good luck figuring it out. While ISO-8859-1 is defined as the default for text/plain I really hope you don't use any high ASCII characters or a European keyboard. A lot of web servers won't automatically add a charset to the content type. With HTML you can define the document's charset in the header of the document itself. Plain text also has no semantic elements so the browser can't make good formatting decisions in absence of explicit formatting instructions.