My partner's one-person service business sends out monthly availability emails to her small list of eager customers. Plain text would not suit her image and brand at all. You are saying she should send a plain text email with a link to a webpage? It sounds like you have no understanding of a business like hers.
I’ve learned over the decades, that “being right” is rather irrelevant when it comes to communication patterns. Language changes and communication patterns, technologies, and how they are used constantly evolve (for better or worse).
While I also would prefer email to be text only, it seems most of the world feels differently. So I stopped worrying about it.
Side note: In theory an HTML email can be more visibly respectful of privacy while being nicely formatted, than visiting a webpage, although in practice that distinction seems increasingly rare.
With you on that for sure, but after years of seeing it the return on HTML emails good and bad is still just too good to ignore. Still great marketing return etc. And with mobile having evolved the way it has and even browser engines/mail clients becoming much more adept, HTML emails (designing them obviously still painful but it's marginally better than 10 years ago), are still working for millions of ppl. And I suppose that leads into some of what this deal is about (other reasons being integration and data etc)