> Kind of interesting that this doesn't appear to have explicititely happened very often before with email software -- but intuition might imply that it should have.

It most certainly did.

Pine

Mutt

Eudora

All are basically done, and are maintenance only. Pine turned into Alpine which added unicode, and that's about it.

There is actually a lot of software where this happens. Tex, and gzip come to mind immediately. I'm sure there is more.