Yeah, it wasn't open source. Edit: wikipedia says it was only open sourced in 2002.
Incidentally, the 'crossing the chasm' approach would be to find some niche where a new OS could succeed - perhaps something where the network effects were less pernicious.
I think you're right. Plan9's first "public" (to universities only) release was 1992, when Linux was just getting off the ground and the FreeBSD & NetBSD projects did not yet exist. People interested in Linux or 386BSD at the time would have probably tried Plan9 if they could have.
Also, does it solve any particular problem in such a way that it can't be ignored? Doesn't seem like it to me.