I assume, following Marx, Durkheim, Berger and Luckman, Bourdieu, and a host of others, that cultures try to reproduce themselves in part through the institutions they contain. Educational institutions are certainly dominant in social reproduction.
So, obviously, is language within the educational project. I'm going to skip the intermediate steps in this thought experiment to get to our required writing programs.
Imagine "college level" writing/language/thinking as a part of the social reproduction project--and of course of required writing programs that may very well exist primarily to haul in profits by requiring courses farmed out to part-time rather than full-time teachers (the excuses for this are legend and more than a little leaky).
But let's say that most people can write well enough to satisfy the exigence of the particular rhetorical situation. If we can ignore concerns like comma splices, split infinitives, subject-verb agreement, and so on, most of which exist only in order to mark one's social class and schooling, most people can write messages that can be adequately interpreted to convey the necessary message.
I--as I assume do other writing program directors--have multiple examples of essays graduate students have brought to me in frustration, saying they couldn't understand them, when if read aloud, the text was perfectly understandable if you ignored the spelling, grammar, and punctuation errors (see Williams, "Phenomenology").
Here's the thought experiment: suppose we accept all writings samples as normative. Suppose we resist the social reproduction agenda of gatekeeping and certifying through the reductive mechanisms of grades, obviously privileging the middle classes who have by and large defined the norms in their own images. What would our writing classes look like?