I believe there are also plenty more nations who are witnessing the same thing, and more to follow. — Christoffer

Yes, here too.

There are no long sentences and paragraphs in the same way as previous generations used in writing. — Christoffer

True. When I read your argument, I thought about encyclopedias. Although it is true that Wikipedia—and the Internet altogether—is a great invention to have access to information, I think they limited the people to go deeper into every topic. For example, I want to know some details about Sweden. If I searched on Wikipedia, I would discover basic info such as the capital city, GDP, extension, etc. But if I decide to take a book of Scandinavian studies, my knowledge about Sweden will be deeper, better, and higher in quality. Sadly, it seems that people only want to focus on the surface.

They don't write letters, they don't write posts like this, no actual arguments or stories; they write in short forms that themselves consist of even shorter elements, previously emojis, but now also acronyms and made-up words conveying specific meanings spread as word-based memes (not just visual ones). — Christoffer

Believe it or not, I still write letters often. It helps me to keep up with grammar and creative writing in my own language. I don't have anyone to send those. I remember that I wanted to send a letter to Alkis Piskas—a TPF member. It was fun. But I see a lot of difficulties in actually getting my letter to be sent to Greece. It is not impossible, though. On the other hand, I think emojis are a good internet tool. I like to use them—you can perceive that I use them a lot on TPF.

What exactly is going on here? — Christoffer

It comes a lot of things to mind, but I personally believe that reducing the effort in education drove us to the current scenario. Some believe that studying a lot of hours is bad for children. Others think otherwise. I think that grammar and complex readings are crucial for a child when he is learning. Instead of continuing watching anime or cartoons all the time, they should read texts and do poetry. But for real. Not just to pass exams. If I were a professor, I would evaluate more the grammar than the content itself. Maybe a student is great in math, but if his grammar is terrible, I think he should not be able to promote. Simple.

Furthermore, let's be honest. People always valued science over language. It is a terrible mistake, in my opinion.

What in culture and education fails to form these abilities? — Christoffer

We—the millennial generation—are guilty, not just education and culture. I would like to know if you were thinking about a private or public educational system, or if this is not relevant at all.

Several things happened.
Before the internet, children were dumbed down by television. While there was some educational and socializing content, most children's programming was purely for amusement, and much of it was anti-intellectual. The more time children spent looking at moving pictures, the less they read.

Before the 1950's, American schools were quite strict and punitive. During the 60's, the power of administrators and teachers was curtailed. To a large extent this was necessary to prevent harsh treatment of children; OTOH, it also reduced student discipline. A number of innovations were tried at that time, some more effective than others, but they generally allowed students to move on to the next level without having fully mastered the basic skills. Within a year or two, the weaker ones would be hopelessly out of their depth, just marking time until they could legally quit. Some of these potential school-leavers were then diverted to vocational programs - or entire separate schools - where academic subjects were neglected.
Meanwhile, teachers had classes of 35 and more students, due to the post war baby boom; they were required to take courses in the new methods in their spare time; they were expected to lead extracurricular activities and supervise lunchrooms, schoolyards, sporting events and dances, and their routine paperwork tripled inside of a decade. When were they supposed to provide extra help for the slower students?

As the general population's reading and math skills declined, news and public affairs outlets adjusted their vocabulary, the structure of their articles and the level of detail in their reports. Over time, information was gradually reduced to generalities and sensations. Schools, too, had to lower their standards in order to keep promoting students, up and out to make room for the new ones.

Since states are in charge of setting curriculum and administer the main funding of schools, poor states and poor neighbourhoods have poor public schools. Additionally, as the standard of living of low-paid workers stagnates or declines, parents have less time to spend with their children; there is little privacy in cramped homes to do homework, and books are generally absent.

As the religious factions push for less science and more scripture; conservative local governments and school boards ban or reject more and more books, and forbid the discussion of a range of disapproved topics, bar critical thinking instruction and unrevised history courses, there is a homogenization of thought which doesn't require analysis or comprehension of complex ideas.

A polity that thinks in slogans and jingles is easier to control than one that arms itself with facts.

It seems to be more than just lacking high level communicative capabilities. That they are unable to form complex thoughts and reasoning.

What exactly is going on here? What in culture and education fails to form these abilities? — Christoffer

The intelligence of a collective group or population can change. This has been discovered by historians. Reading comprehension, depth in understanding, and ability to construct complex written or spoken narratives can be undermined by technology, among other things.
So, then the question becomes, can a reduced intelligence be the cause of a downfall of a culture? Yes! We've seen over histories that cultures/kingdoms had risen, reached their glorious era, then vanished.

If I were a professor, I would evaluate more the grammar than the content itself. Maybe a student is great in math, but if his grammar is terrible, I think he should not be able to promote. Simple. — javi2541997

Where I taught we went through a gentler version of this, grading partly on grammar. But trying to teach both subjects simultaneously was counter productive. And I am not sure the ability to write lengthy pieces with delightful descriptive flourishes is an admirable trait where succinct, to the point passages would suffice - in fact, be more readable. For example, many long convoluted posts made on this site.

But trying to teach both subjects simultaneously was counter productive. — jgill

I understand, yet I was referring to a special group of students. I meant the ones who are around 17 or 18 years old, and they are nearly finishing high school to enrol in university. Some of the are good—or even brilliant—on maths, literature, philosophy, physics, etc. According to their skills, they would choose one degree or another. But, no matter the degree, I think spelling is very important. At least, writing with proper grammar is a proof of non-illiteracy, in my honest opinion.

And I am not sure the ability to write lengthy pieces with delightful descriptive flourishes is an admirable trait where succinct, to the point passages would suffice - in fact, be more readable. — jgill

Yeah, brief texts could be better and more comprehensible than long ones. Nonetheless, if we want to teach young people critical reading, large texts are also important. Furthermore, I think it is essential to teach them to not read only the surface of every topic. For example, I am not an expert on mathematics, but some paradoxes are interesting, and I want to expand my knowledge of that. Wikipedia would help me; that's true, but I bet there are clear papers around the Internet that could be more complete.