I'm pretty sure that this is just a lower-strength version of "the wiki trap" mentioned in the article - if you write all of your pages in HTML, then they're going to stay HTML unless you invest the effort in writing a parser+translator that converts all of your content into Markdown or Wikitext or whatever else you might want to migrate that content into.

For instance, if you decide you want to give a templating language like Mustache a try - have fun programmatically extracting your content from those HTML pages and translating it into Mustache. Doable? Sure. Easy? Certainly not - and it's going to be very painful and manual if you didn't keep the same format for all of those HTML pages.