I work for one of those “evil” email software companies. Part of our business is external marketing and the much larger part of our business is internal employee communications. For over 15 years we kept track of the number of people who used HTML emails and the number of people who opted for plain text emails. It was something in the realm of 0.1% of all mail recipients had opted for plain text. That was in 2015. Today, that number would probably be even lower. So we stopped offering multipart emails entirely because the cost/benefit analysis doesn’t add up.

HTML emails aren’t sent only because that’s what the senders want. They are sent because that’s what the recipients want.