Digital content longevity will continue to be highly variable, depending only in part on the file format used. HTML has existed for about 27 years and I wouldn’t venture a guess to say how much longer it’ll go. I can say that a reduction in ceremony around opening and reading a file is better for that file’s longevity. Relatedly, the ubiquity of software necessary to read a file lends to its future proofing as well. And what software has been historically and continues to be more ubiquitous than the lowly web browser? I’m not sure such software exists.
So feel free to keep creating your content in Microsoft Word or in Markdown or using JSX or Mustache templates or in your WordPress database. But if you want the content to live a full and happy life, publish it in HTML on the web.