Don't get me wrong, I'm a big fan of plaintext email (and use mutt at work) but it's not practical to expect the world to bend to your own narrow choices in technology so harshly. The attitude displayed here is, quite frankly, disgusting.
Just because we invented and pioneered the early 'net doesn't mean we own it. Things change and usage patterns evolve as people join and leave the medium. Email today is a medium for HTML and emojis for almost everyone using it, except for the curmudgeons. We don't get to define "communicates properly".
Don't be a curmudgeon. This isn't "our" system anymore, if it ever was at all.
Within a given organisation (if you're exceptionally lucky), or on some mailing lists, you might be able to enforce discipline. At large, though, this mostly doesn't scale.
I think it's unfortunate -- there's a very strong degree to which standards of message formatting are useful and helpful. And, in counterpoint to much of the disagreement to your point, it's not as if a bunch of Unix geeks in the 1970s and 1980s started the concept -- go through any "how to succeed in business without really trying" manual from the 1920s through 1970s, and you'll find strongly specified guides on how you are to type and format your business correspondence (using your monospace Royal, or later, IBM Selectric, typewriter). And deviations from those standards were not appreciated.
But until and unless some Hand of God comes down and enforces discipline again, we're in our current State of Babel.
On the other hand, client quirks are so common now that configuring you email stream to:
1. Render all incoming messages as flat, 72-character, ASCII
2. Outputing messages with same, '> ' quoting, etc., etc., is pretty much perfectly justifiable.
"Oh, my email doesn't handle attachments or binaries, I'm sorry."
If you are in a position to ensure people talk to you on your terms, so much the better. I can approach that, but there are sacrifices to be made.
My approach is to go as far as I must to meet them halfway because I know I wasn't born perfect either.
Engineers are brilliant in ways that are rare, valuable and admirable. But they don't tend to have a great level of empathy and trust me, it's something worth cultivating.
I've heard people complaining about email conversations that they can't understand any more because every one used together top-posting, bottom posting, coloured inline replies and no trimming.
I want this madness to end. If it isn't empathy, what is it?
It's no different than telling billion people to stop using Flash or kiss a specific part of my body and rest peacefully. Except perhaps that we are a few anonymous "disgusting elitist curmudgeons" instead of being huge internet companies.
The problem isn't HTML, it is its users. People who can't quote properly, people who end every single sentence with an emoji, etc. In a nutshell, people who have never experienced Usenet never learned how to communicate properly with a keyboard.
The fix for this is to configure your email client to auto-answer a rejection message when it receives anything other than plain text. You can always blame your antivirus for that.
Help people with disabling HTML email. With a little bit of luck, they won't remember how to enable it again. And hopefully, since they won't be distracted any more with fancy formatting, animated smiley and which GIF-meme-to-use-for-this, hopefully they'll focus on words and reinvent ascii art.