Mobile is often less than that and any hard wrapping looks absolutely horrid. I mean, hard wrapping in general looks disgusting on anything besides an ancient VT-80, but it looks extra bad on mobile. Only tolerable option for plain text is "format=flowed".
Hard wrapping is nice. My screen is too large for any kind of flowed text so I often have to add the extra margin that is required to get back between 80 and 120 characters. On mobile, you can just rotate the phone.
I've had autorotation turned off for a decade, I'm not turning it on for your email. It's a bit odd that plain text proponents don't want HTML imposed on them but are okay with imposing this.