People who preen online about their command of English grammar often assert that they see a general cultural and educational decline, most frequently calling it a "dumbing-down."
In a forty-year career as an editor at newspapers, I have edited the work of people twenty and thirty years older who received, as I did, the traditional instruction in grammar at school, and of people twenty and thirty and forty years younger who received little or none of the traditional instruction.
And every day for forty years, I have sat down at my desk to deal with the same things. The same damn things: subject-verb agreement, misplaced modifiers, incorrect homonyms. All of it, mind you, the work of college-educated journalists whose profession is writing in standard English.
For that matter, my classmates in the public schools of Fleming County, Kentucky, in the 1960s do not necessarily do any better, despite their exposure to the traditional teaching of English grammar.
The traditional method was not particularly effective, and it left a bad taste in the mouth. Some did learn from it, as some will learn something in almost any pedagogical circumstances—Dr. Johnson believed that boys could not learn the classical languages unless they were beaten.
The British linguist David Crystal writes in Making Sense that "the negative associations that surround grammar are the result of unhappy learning experiences, in which complex sentences, artificial examples, pedantic rules, mechanical analyses, and poor explanations have combined to produce a penitential mindset: 'Grammar is good for me, and if it causes mental anguish, then so be it,' "
That people could develop a solid grasp of formal English grammar under such unpromising circumstances is a real accomplishment, even though, as you can read in Bad Advice, a great deal of what they remember is unreliable.
So let's not make proficiency in grammar, the grammar of formal English, which was badly taught for decades, and then not taught at all, a measure of individual or national intelligence.
Speech comes naturally, but writing has to be learned, and most people never get very good at it, particularly in the dialect known as formal written English. We can see that online, where anyone with a computer can become a published writer. As Gretchen McCullough writes in Because Internet, we can look beyond edited publications to see how people actually write.
From there we can surmise that people in general are about as dumb, or intelligent, as they have always been. We can further surmise, from internal evidence, that the "dumbing-down" trope is trotted out when the writer merely wishes to establish a superior social class standing. That is when the reader will recognize that it is time to move on.
There's a difference between cache and cachet, but knowing that does not confer cachet.