Ok, in some cases I see why html would make some sense. In those cases, my opinion is: send text and invite the person to view markup content in their browser.
The e-mail context is not suitable for markup, styling, and all this stuff.
Another problem are mails that don't have the TXT part and *only* HTML.
I plain cringed when I saw the headline. This sounded like a tool designed by some evil entity to bring destruction upon the World.
The problem is there is no practical way of getting back to plain text, or even to a different type of markup.
To say nothing of legitimate marketing emails (which many people rely on to get coupons and deals) that has to feature pictures and the like.
None of that requires HTML. Using ">" to denote previous conversation quotes is the standard and they normally get highlighted different than reply text. And images just need a decent 'drag-to-add-attachment' flow.
Marketing emails don't matter as they are an abuse of the platform.
Handing down diktats about the correct use cases and workflows for a flexible thing that is currently being used by billions of people in billions of ways - like email - will either reduce your personal relevance to them, or, if you succeed in pushing your vision onto the popular thing, drive people away from it.
I don't think there is such a thing.
The senders of those emails are happy to send them.
Unless you - an unrelated third party - think you have the right to insert yourself into this business relationship, it's completely legitimate.