[Paulomus]

I think it is significant that both Unix itself and Linux were category killers. Unix succeeded because people kept going "Wow, there's nothing like this!" and writing in to get it, copying it passing it on to other people. When Linux took off, there was no Free-ly available Unix-like OS (at that time). Linux quickly gathered a critical mass of users who saw it as something like the answer to their prayers. I guess the strengths of Plan9 were not such as to create a new category for itself, and its flaws made it an unsuccessful competitor against existing Unix-like systems.