In other words, had Unix been a closed proprietary OS,
would we even be
talking about C? Or would it be sort of a vintage curiosity like BCPL or
B?
If you look at IBM as an example, many (most?) of their languages went on to
fame both on their own platforms and off. FORTRAN, COBOL, and PL/I are the
best examples. None of that was ever open source in IBM land and they're
all still popular.
If you are saying the popularity and pervasiveness of C is primarily
because it was used to write UNIX (and then Linux), I would certainly agree
with that.