This is all very nice, but the fact that the article doesn't even _mention_ images or attachments shows how divorced it is from the reality of most people's email experience. I happily used emacs as my mail-reader from 1989 to about 2009 (first RMAIL mode, then VM). The increasing importance of non-text formats made it cumbersome, and I somewhat reluctantly switched to GMail. It's hard to imagine switching back now.
the only images i find are either from spammy adverts or tracking pixels. Normal people don't put images in emails. As for attachments, you can open them in their native programs just fine.
Normal people send me pictures in emails all the time. Very often it's a screenshot of something where they have drawn a circle around some part and written some variation of "this part is wrong" or "can you take a closer look at this".