HTML emails are mainly used for marketing - that is, emails you probably don't want to see in the first place. The few advantages they offer for end-users, such as links, inline images, and bold or italic text, aren't worth the trade-off.
The idea that marketing emails = "you don't want them" is fairly popular in some crowds, but it's contradicted by the tremendous business success of using marketing emails. The reality is that most people want and use marketing emails.
For example, I'm signed up for marketing emails from several airlines and these routinely save me money when booking vacations. Cheap airline tickets are a limited resource, so real-time notification of new availability has financial value to me.
Same with end-of-season sales for clothing I like. One company gives email recipients 1-2 days to shop before the sale is posted publicly on the website and social media.
It also ignores the tremendous popularity of email newsletters, which employ HTML formatting to improve the user experience, exactly the same way content websites do. In fact HTML email newsletters are often better than websites, because email clients don't execute javascript. The advertising is far less intrusive.
but it's contradicted by the tremendous business success of using marketing emails
I know advertising is effective, it IS manipulating me. That's WHY I don't want it. "I don't want them" is not contradicted by the success of marketing, it's reinforced by the success of marketing.
Does it matter? The default is HTML, the majority of mail is HTML (or atleast multipart). So why does it matter if GMail defaults to what the majority is doing? Everyone (99.9999% of people) can receive and send HTML, so just do that, it's the informal standard for mail now.
It's great that you think that it matters but I don't think that what you think that it matters matters in practise, really, the world is HTML email now.
The people you are interacting are not sending HTML email on purpose, but simply by accident because of details. I would hazard to guess that if the default was changed that they would not noticed.
Given all the communications that you receive, how often have typographic 'flourishes' been added in a useful way that would need mark up more advanced that ASCII/Unicode?
Given the following (from the article):
* HTML as a vector for phishing
* Privacy invasion and tracking
* Mail client vulnerabilities
* HTML emails are less accessible
What exactly does adding mark up give you on a day-to-day basis over a text/plain Content-Type?
HTML emails are mainly used for marketing - that is, emails you probably don't want to see in the first place. The few advantages they offer for end-users, such as links, inline images, and bold or italic text, aren't worth the trade-off.
and more at https://useplaintext.email/#why-plaintext