alt Hacker News386BSD -> FreeBSD
NetBSD -> OpenBSD
To some extent I would argue that Linux is a spiritual fork of Minix and Unix.
But I think it’s also worth pointing out that these were in vastly different times where a single person or a small group of people could maintain an OS. An OS or browser is vastly more complex now.
Network effects were also nowhere near as strong as they are now.
BSD is a killer example of that point, too. Using a BSD Unix got to be pretty difficult after Linux containers became hegemonic.