The former listserv users only want to watch their categories. They would absolutely lose it if they got every email for every category.
The site will cover 40+ areas of law and most lawyers only practice in 2 to 3 categories. Would apply to other sites with a variety of distinct subject matter areas, geography, etc.
As discussed on meta, the mailing list mode is the nuclear option. Not sure users should have that option except in limited special purpose sites.
Eventually these users will figure out email doesn’t scale. Until then, they want their email.