Projection editors largely seem to have failed in the past because they are overwhelmingly focused on programming languages, where syntactical editing reigns supreme and "worse is better" is also an important concept. Markdown, for example, is not as complex as building a projection editor for html.
Even Lighttable failed. Lighttable was an interesting attempt at a new IDE that allowed you to do various things, including search documentation, view values that flowed through a Clojure program, and operate on a function level.