Again, I think it's important to remember that in 1979, the G rating did not have the "kids' movie" connotation that it has today. It was used for films that were suitable for family viewing -- movies that didn't have anything parents might not want their kids to see, like sex scenes or gore, but were not necessarily aimed at children. That's the case with TMP. It was meant to be a sophisticated science fiction film with adult appeal; it was simply kept free of content that might make adults think they had to leave the kids with a sitter when they went to see it.
I agree with the end of your statement, but I remember quite distinctly that by the mid-1970s (and certainly by 1979) the G rating was almost always used either for Disney and other kid oriented movies, or revivals of older films (like Gone With the Wind
). Even E.T. (released in '82) was given some off color dialogue to insure a PG, so not to scare off teenagers adverse to 'baby stuff'. The period when the G and X ratings were neutral was the late 1960s and early 1970s, hence serious films like Midnight Cowboy
and Clockwork Orange
getting X's. By the late 70s, the X rating (as well as the unofficial 'XXX') was reserved for pornography (most of which weren't even submitted to the MPAA for rating).