I don't understand the fixation the industry has on multi-player development either. Zed isn't the first. It's been 10 or 15 years since I started seeing multi-player development (sometimes called peer programming in an earlier iteration) being championed in various ways. VS, Intellij, Eclipse, and vscode had plugins for it and all of them are from the 2015-2018 time. One can even say that the Google Wave demo from 2007 was one of the earliest demonstrations for this concept.

I even worked on a team where one engineer loved the idea and championed doing it as early as 2013. Most of these IDE extensions didn't exist back then. He ended up picking a buddy, and they moved to the same office, hooked 2 keyboards and 2 mice into the same PC and spend literally the entire day alternating who's writing code in the same IDE.

I think the concept was supposed to be about helping developers share their skills. Every developer has an experience of looking over somebody's shoulder and having an "oh shit, I didn't realize there is a shortcut/tool/command for that" moment. And the idea with multi-player programming is to get you to see that and learn from it more. But. It. Doesn't. Work.