&gt; it looks like Plan 9 failed simply because it fell short of being a compelling enough improvement on Unix to displace its ancestor.<p>I really don&#x27;t think so. Unix caught on because it was (initially) available for free with source code. It was freely available at the time because the Bell System, as a regulated monopoly, was prohibited from getting into the software business. Universities loved it as an object of study, and it spawned many derivatives, also freely available. After the breakup of Bell, they started marketing Unix.<p>Plan 9 in contrast was not released for free, had restrictive licensing, because Bell Labs was now under AT&amp;T. I think it is hard to overstate the impact of this difference.<p>Linux became popular first because it was free, and because it invited hacking, and then because the combination of these caused a snowball effect.<p>I would like to add that Plan 9 is in fact very compelling, but the advantages are maybe hard to appreciate. A big issue was also that it initially supported a limited set of hardware because they didn&#x27;t use the hardware BIOS.