Good question.
I just went through one of my backups. Recently they came up with TextBundle:
http://textbundle.org/ and they're using it in the iOS version, and maybe even the Mac (I don't have it yet).
Probably will have a go at that actually.
I guess I'd rather deal with having to do this
now, then figure out how to read WYSIWYG files ten years later. There's lots and lots of threads where people can't open old Word files, etc.