> I think this result needs more investigation. We need to know why people open plain email more than those fancy designed ones, rather than just having those numbers.

I've seen plenty of people who make quite an effort not to receive/open anything that ain't plain text for simple security concerns. Similarly, many design emails end up being displayed butchered for users due to adblockers/email provider/client blocking outside resources and links.

Imho: If I want a website like experience I can visit the website, emails should be reserved for simple text communications, without adding needless design bloat, but that's just my personal preference.