We've done the opposite. After ten years in operation we've only just begun to add html parts to our emails. Its just easier to render plain text, and no one has the time to be spending hours templating and testing emails, we've got better things to do.
And the really amusing aspect of that is that text-focused emails (be they text/plain or deliberately simple and barely styled text/html) tend to get better engagement than fancy branded emails—the data I’ve seen from a variety of sources is quite conclusive about that.
To a large extent, hard wrapping is a combination of choices made by the sender (to hard wrap the text, because it doesn’t trust clients to soft-wrap properly) and receiver (to soft-wrap or not).
But even without that, the quoted-printable content-transfer-encoding means that the hard wrapping isn’t actually necessary—you can have lines as long as you like. Some email clients do thereby send text/plain with arbitrarily long lines, which will cause overflow in some clients (typically where plain text is shown in a monospace font), and be wrapped in others (most, these days, I think; these are typically where plain text is shown in a proportional font).
Then too, some email clients ignore the 78 character limit (it is, after all, only a SHOULD [1]) and write longer lines in the MIME message. (Not sure what they do after 998, which the spec says they MUST not exceed [1].)
But the inability to specify the desired semantics of proportional-with-wrapping versus monospace is probably the part of text/plain email that makes me saddest.
