I’m just going to respond to a couple of specific points here, to avoid going around in circles with repetitive arguments.
I disagree and I think this mischaracterizes the argument I’m actually making. Sure, “rules” like never ending a sentence with a preposition (which I think someone mentioned upthread) fall into this category, but that’s a classic straw man because it’s not actually a rule that any sensible person would make or care about.
What I’m talking about is a consistent pattern of bad writing, full of convoluted sentence structures, bad grammar, spelling mistakes, awkward punctuation, eggcorns (“for all intensive purposes”) and all the other familiar markers of marginal literacy that we see so often. The problem with this sort of bad writing is not that it violates some contrived standards of cultural etiquette but that it’s problematic for entirely practical reasons, because it undermines the essential purpose of language which is effective communication.
The argument that it’s perfectly acceptable writing because you can, in fact, understand it is rather specious, somewhat like arguing that an unmarked, rutted, boulder-strewn dirt road is just as good as a well-marked freeway because it will ultimately get you to your destination. It may, but it’s a slow and bumpy ride, and you may get lost or bust an axle on the way. I’m certain that if you measure the time it takes a reasonably literate person to read well-written prose compared to the time it takes to struggle through semi-literate composition, the latter will take significantly longer, and will be much more susceptible to misunderstanding. It’s why reading bad writing leads to a sort of mental exhaustion – it’s hard work!
Yes, this is indeed exactly what @Left_Hand_of_Dorkness was referring to. But the fact that we all have an innate instinct for language and a deeply ingrained sense of the most rudimentary aspects of grammatical structure is hardly the point here. The ability to say “Me like sammich. Sammich is good. Me want more.” is not the basis of a successful civilization or a productive society.
Those who would dismiss this as mere classism should have a look at the article cited below. It suggests that low levels of literacy in the US cost the economy an estimated $2.2 trillion a year. It contains the following rather astonishing statistic (bolding mine):
According to the U.S. Department of Education, 54% of U.S. adults 16-74 years old - about 130 million people - lack proficiency in literacy, reading below the equivalent of a sixth-grade level. That’s a shocking number for several reasons, and its dollars and cents implications are enormous because literacy is correlated with several important outcomes such as personal income, employment levels, health, and overall economic growth.