> they're able to understand and be understood sufficiently

Communication isn't all or nothing. The more expressive power you have, the wider the scope of ideas you can communicate. It may be minimally sufficient to be able to communicate "me hungry" or "that crazy", but more is probably still better.

To add to this, expressive power also makes it possible to communicate nuance and discuss biases and fallacies.

I should have made my point clearer that I believe low literacy rates hurt your ability to articulate yourself and to understand others and even engage in basic reasoning well outside the written word, and that this is (often) detrimental to your ability to navigate the world. I certainly enjoy engaging in memes and jargon and slang too—in fact it's a great joy in life to find new words and manners of expression and to look beyond what worked yesterday—and much of this is just as expressive as formal or classically-analytical english, if not more so given the baggage words tend to acquire over time. But that's not what I was referring to at all.

I should have made my point/question clearer too; I agree that literacy is valuable for all the reasons you outline.

Tests are only as useful as what they measure. The qualitative changes to communication could have impacted the quantitative changes we're seeing (i.e. lower literacy rates). How much, in what ways, I have no idea; I was hoping to learn more by sparking discussion.

Unfortunately (and ironically) my poor choice of words has instead led to my apparent taking of an anti-literacy stance. Oh well.

“Don't you see that the whole aim of Newspeak is to narrow the range of thought? In the end we shall make thought-crime literally impossible, because there will be no words in which to express it. Every concept that can ever be needed will be expressed by exactly one word, with its meaning rigidly defined and all its subsidiary meanings rubbed out and forgotten. . . . The process will still be continuing long after you and I are dead. Every year fewer and fewer words, and the range of consciousness always a little smaller. Even now, of course, there's no reason or excuse for committing thought-crime. It's merely a question of self-discipline, reality-control. But in the end there won't be any need even for that. . . . Has it ever occurred to you, Winston, that by the year 2050, at the very latest, not a single human being will be alive who could understand such a conversation as we are having now?”

> The purpose of language is communication

It isn't this simple. Literacy plays an important role in information assessment and in decision making. It's strongly associated with health, success in personal finances and career path, and relationship satisfaction, to name just a few areas that have published papers readily accessible online.

To get a look at the ugly side of this, high demand religious groups are a fascinating case study. Constant circulation of repetitive cliches is a key mechanism for isolating their members and making them easier to manipulate.

It matters because of entropy. How many people can society carry before it suddenly can’t? Civilization doesn’t persist from inertia.

the bar of being a productive member of society is much higher than "sufficient communication" (which I would argue the illiterate aren't capable of anyway)