It's kind of odd to call C a general purpose programming language today. When it came out, it was, just as it was a "high level" language, but would not be considered so today. These terms tend to be relative to other commonly used languages.
I would probably say "While the goalposts are always moving as to what might be considered a 'general purpose programming language,' Objective S is, to my mind, the furthest in that direction so far."
Wordier, but should rub fewer people the wrong way.
Actually I have books that call it middle level language, because basic stuff like strings and arrays are Assembly like versus something like Modula-2.
I would probably say "While the goalposts are always moving as to what might be considered a 'general purpose programming language,' Objective S is, to my mind, the furthest in that direction so far."
Wordier, but should rub fewer people the wrong way.