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I agree Plan 9 is not a good example in this story. Plan 9 didn't failed because of the problems associated with a re-write as described in Joel's post, but because of political and social issues.

> Research Unix V8, V9, and V10 got virtually no uptake, despite being incremental evolutions of the original upstream, due to a mixture of inertia and licensing restrictions from AT&T.

I'm not sure what you mean by inertia. v8-v10 were not widely released to avoid another BSD incident.

v8-v10 were 4.3BSD derivatives, so I'm not sure I'd call them incremental evolutions on the original upstream. They removed and/or reworked all the new BSD stuff though, and the basic userland was very custom. Some of it survives (in some form) in Plan 9.