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.
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.
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.