Plain text is plainly unusable for a significant amount of people. Mobile? Have fun with a small font size to accommodate 80 chars. Ultra-wide? Same flowing problems. Right to Left language? Learn to 'work' with zero-width characters or you're SOL. Want to give more data to screen readers (sometimes the table is actually a table)? Hah.
Some people need to understand there are more use cases to email than mailing lists and that the 90s aren't coming back.