I learned how to program in MUDs, made friends (including my first girlfriend, online dating was very different 35 years ago), and (mostly) had a great time. The death threats weren’t much fun, especially since he lived 2 hours from me.
I am curious about your “biggest sellers” comment. What was sold, and by whom?
I'm still living with someone I met on a MU.
And yeah, text adventures* were some of the biggest sellers, not mus. While there were* a few pay mu*s most of them were hobby projects.
My guess would be Zork and the other Infocom games which at the time were huge because they could be played on the very limited PC's of the 80's and were top 10 selling games. Myst is basically a graphical "text style" adventure that was huge in the 90's and drove lots of CD-ROM sales and was a top seller for like a decade.
MUDs were fun because, as a kid who read various magical fantasy series, there would inevitably be a server that was spun up attempting to recreate the world and magic system. Because it was just text, it made it far easier to do fan works like that. It's something kids today probably can't imagine. Really enjoy Twilight? You can join as server and make a character who's a [N]ormal Teenager, [W]erewolf boy, [C]enturies-old Sparkling Vampire. And you'd have a whole list of skills and progression for 100 levels. Wheel of Time had several different competing codebases with differing versions of channeling.
It was an entertaining way to learn C as a kid, and programming new weird spells made one feel like an actual wizard.
I think there's still some space for MUDs in the world. You could build a programming/literacy class around just teaching kids how to navigate and interact within that world, how to be a god/mod/admin within the world using its tools, and how to modify it and compile it with their desired changes.