You send me anything other than plain text in an email, and I don't already have a source filter for your address, it gets sent to a folder that I might look at if I have the time some day.
Most such mail tends to be not just HTML but rather multipart/alternative with an HTML but also a text/plain part.
Luckily, HTML only (or its bastard cousin: including a text/plain part that is horribly broken or just says "please enable HTML viewing") isn't seen that often around here.
Normally one does business on the website and then what one gets in their mail isn't an interactive utility running in their mail client but rather either some record of the transaction you will rarely need or a link to complete some activity on their actual website.
Click this link to reset your password or some such.
99% of important transactional email I get is HTML. Kind of nice to have your boarding passes and two-factor authentication emails easily visible.