That's about three levels deeper than most users are comfortable navigating, and as such only a negligible fraction of users will ever see this setting, understand what it does, and change it. Too bad every enterprise solution defaults to HTML anyway.
I wrote out the full path to it, but Preferences is the default page in Settings, and Reading and Compose & Reply are just sections inside it. So it’s actually more like “go to Settings, click Show advanced preferences at the bottom of the page, and search for ‘plain’”. And yeah, it is an advanced preference, whether you’d prefer otherwise or not.
Most users probably don't care about HTML versus plaintext emails. The ones that do care would be the ones willing to pay $0.10 extra per month to enforce plaintext receipt, or willing to dig through a few levels of configuration to enable plaintext receipt.
TricepMail has granular/heritable switching between plain text or HTML, with or without images. For example, if you have a mailbox set to plain text but receive an email that you would like to render as HTML, it's just one click away for either the entire thread or just that message.
