Its popularity and persistence was partly because of its size. Each release got a lot bigger. A v6 system fits easily into 128 KB of RAM and there are strippe

Its popularity and persistence was partly because of its size. Each release got a lot bigger. A v6 system fits easily into 128 KB of RAM and there are stripped down ports that run in less. v7 is just a little bigger. Small and comprehensible. See: https://en.wikipedia.org/wiki/Lions%27_Commentary_on_UNIX_6th_Edition,_with_Source_Code https://en.wikipedia.org/wiki/Lions%27_Commentary_on_UNIX_6t... V7 also persisted well into the 80s because it was a lot easier to port. And again the size meant it could fit on small machines. In 1983, the year the IBM PC XT with a hard drive came out, commercial AT&T UNIX was up to System V which was enormous. It wouldn't have fit in the RAM or disk of the XT.