If you don’t like the emails your electricity provider sends you, don’t give them your email address, or filter all of their emails to the trash.

Rich text emails as a format and as a feature are great. If you don’t like the content of emails that doesn’t change the issue at all. In the alternative you are proposing, the unsubscribe link isn’t clickable at all.

(I also dislike Discord for many of the same reasons, but it won because it is better for users, because it has more features. Your complaints about it, which I share, are irrelevant to this discussion.)