Do you eschew the use of typographical techniques that aid reading, both speed and comprehension, such as emboldening, headings, breaks, italics, font and colour changes?
It is interesting to note that some of the most important works produced by mankind can be reproduced in essence without these aids [1]. Moreover, these well-meant aids are perceived as undue distractions and hence disabled by many [2], most of which are not die-hard plain-text users.
Gutenburgs top projects are Kama Sutra, Sherlock Holmes and Punch - whilst Burton's original translation was text only I think you'd be hard pressed to argue that the first of these has no need of illustration. The second is a [series of] novels, plaintext is fine there though often even novels have images ("plates") of some kind. The third, Punch is probably best known for it's illustrations and it's conveyance of political satire specifically through imagery (eg http://www.gutenberg.org/files/20759/20759-h/20759-h.htm).
Readbility uses background colour highlighting, keeps some formatting such as pulls and retains images.
You're welcome to prefer plaintext, some prefer black and white TV I'm sure, I just find it rather lacking in presenting the diverse types of information I wish to send and receive by email.
The moment the work in question has as many as one image (or video, for that matter), you obviously neeed to leave plain text. My humble and non-representative point is that text is generally best left plain. The written language is powerful enough to avoid all these devices, which soon enough can become distractions.
The question isn't whether important documents can be reasonably be represented as plain text. Clearly many can. The question is whether the typographical techniques listed are an aid in reading. And just as clearly, they are.
And just to note, Readability actually preserves bold, headings, breaks, and italics, while providing a different set of fonts and colors based on the user's preferences.
My bad on Readability, as I made an assumption without checking.
I see what you are saying, and I partly agree, but I am of the opinion that language is rich enough to emphasize relevant points without further aids. But it's becoming SO common to use _SO_ many aids that this is already standarized with two levels in HTML, namely the em and strong tags. How long until we need a third semantic level of emphasis? Of course, I enjoy freedom and choice, so assume people/the market must prefer your style by and large.
For the record, I know that italics typography has been used for centuries as an aid in emphasis. Still, I humbly feel this is an unnecessary artefact, which more often than not turns into a crutch.
I like how you went to ALL-CAPS because you lacked the ability to add proper textual emphasis and then that you felt the need for more emphasis still and again broke writing conventions to indicate you wished to add underlining. Made your case for plain text perfectly.
Buttons for clothing have been invented, pretending that my coat doesn't need buttons and then holding it together with my arms so I don't freeze doesn't make sense to me.
It is interesting to note that some of the most important works produced by mankind can be reproduced in essence without these aids [1]. Moreover, these well-meant aids are perceived as undue distractions and hence disabled by many [2], most of which are not die-hard plain-text users.
[1] http://www.gutenberg.org/
[2] https://www.readability.com/